Hot take but the ancient laws surrounding gods interacting with their kids in pjo only exist because Hestia failed to do her job.
Before you come at me with pitchforks let me explain my theory!
Demigods as children of the gods and heroes fall within a lot of the god/desses domains.
And yet, Demigods that aren't of the god king's seed fall no more within Zeus's domains of influence than any other greek does as Zeus subjects.
A demigods their personal life, especially their early years and childhood doesn't really belong to Zeus.
He's no god of Childbirth, no Protector of Youths or Patron of Heroes beyond his personal champions. The God holds no dominion over Family or Home.
The closest he comes to these is his position as the enforcer of Xenia and a god of hospitality which I think we can all agree isn't the same thing at all.
Greek Myths are full of situations where Zeus stuck his nose into situations he wasn't wanted or needed but the god is also a king a someone who loves his family.
He knows where the lines are, how far to push and when...
The situation between Demeter, Persephone and Hades should have made it obvious if nothing else that it never ends well when he meddles with his siblings and their children.
Never mind the rest of his family.
He may be willing to do so if pushed, but he wont without cause.
This is shown in the books by his choice to (try and fail to) kill Bianca and Nico alongside their mother in order to prevent the prophecy; yet spare Percy in the Lightning Thief for the sake of family unity and not instigate a war.
So why would he craft the ancient laws surrounding Gods interacting with their kids; something that no one agreed with and would actively set most if not all of his family against him; unless he had no choice?
The rule about Gods interfering with Quests makes sense to some extent as something that Zeus would implement.
As the god of Justice, Enforcer of Oaths, Laws, Ruler of their Pantheon...Quests fall within his purview.
Whilst Quests can be amusing and sometimes random, many of them are incredibly serious and often as we saw in the books politically motivated.
The actions of heroes and demigods can instigate or prevent wars and depending on the agenda of the God in question they will seek to prevent or help the quest along.
Removing Gods from the equation prevents them from muddying the waters and allows failure or success to be placed upon squarely on the Questers and perhaps their sponsors.
It's logic for him to implement something like this to reduce the chance of internal wars and conflicts.
Then there's the rule about Gods not being allowed to directly steal each others weapons/symbols of power. This one seems to be part of some kind godly equivalent to the Law of War and the treaties mortals have when it comes to warfare.
Which, again, falls rather firmly within his domains of rules, law and possibly even hospitality as even enemies were expected to follow Xenia.
Frankly the ancient laws seem to be a complex web of peace treaties, rules of ethical warfare and diplomacy with demigods sprinkled in to act as the necessary loophole/s to fix a situation before it implodes and brings the entire system crashing down.
But again the laws surrounding raising their kids seem to be an exception to this assumption and I would go so far as to say that they're one of the reasons the whole house of cards keeps coming close to crumbling -> Something exemplified within the books and how many demigods sided with Kronos.
His decrees surrounding childrearing have little if any support from the other gods except Athena and maybe ambivalence from gods like Ares who prefer a hands off approach (and who knows if their acceptance/tolerance is something that they always held or a recent development that came about from modern interpretations of myths warping their behaviour).
If anything the gods opinion on it seems to be equally poison as the ones held by their children at times or even worse in some cases with many of them actively trying to find work arounds to be with their children.
Zeus may not break the rule but we know from interactions with i think it was Jason? in his Jupiter form that the only reason he doesn't is because as King he must hold himself to a higher standard and live by the very rules he decrees.
From this we can determine that the rule isn't even something Zeus/Jupiter is happy with.
Realistically the raising of demigods isn't something Zeus should want to touch with a ten foot pole.
So, again, why did he create it?
He's paranoid and suspicious and often appears cruel but again, he's no fool.
It doesn't make sense for him to implement such a rule and place himself in the position of being hated for it by his family (and most of their pantheon) unless all other options had been exhausted.
I cannot see Apollo and Artemis, Heracles or even Hera disobeying a direct decree from their Father/Husband/King to become more active in their domains and focus their efforts on demigods.
Protectors of Youth, Marriage, Childbirth, Maidens, Heroes, Defender Against Monster... all of these domains relate to the issue the ancient law in question seems to address but they aren't the core of the issue.
The domain that is primarily Hestia's as the goddess of Hearth and Home.
By all accounts if there was an issue in this area then it would have/should have been Hestia's responsibility to handle it.
Now, its entirely possible that the Goddess tried and failed before asking her brother to intercede as King. Something she would have had the right to do.
However, given her behaviour in the books from: the pacifism that had her sitting out on the war that had literal children fighting and her siblings facing down seemingly certain annihilation at the hands of Typhon; to the refusal to get involved in any conflict as she just chose to watch on as an outside observer as her family tore itself apart (call back to her giving up her seat to Dionysus instead of having him, Hades and Persephone join to keep up with the power of 3); and just her lack of involvement besides seeming to act as the occasional comfort to other characters (yes she's present at camp but most campers apparently can't see her as she sits by the fires and honestly it makes more sense for the goddess of Hearth and Home to be camp director and yet even before Mr. D punishment she held no such title and seems content not to).
It would not surprise me if Zeus had asked/ordered her to get her shit together and she just... didn't.
(there is a reason I agree with @bugwolfsstuff about Hestia's fatal flaw being inaction, it seems entirely in character for her to see the family falling apart, the other gods behaviour when it comes to their kids becoming toxic and dangerous to everyone involved and just. not. do. anything.)
After all, what is Zeus going to do to the most beloved and favoured goddess in their pantheon? The other Kronide's, never mind the rest of the Greco-Roman gods would rebel if anything happened to her.
So, instead of the issue being handled by someone suited to it, Zeus does his best to fix it the way he handles most problems: strict implementation of blanket laws that effect everyone equally regardless of how concerning or problematic their behaviour may or may not have actually been.
So, yes this theory is largely conjecture but it makes sense in my mind.
Zeus is an ass but we cannot blame everything wrong with the gods in pjo on one man that gives him both too much and too little credit for the wrong things and doesn't recognise that the other gods (despite their childish behaviour) are grown entities who need to be held accountable for their stupid decisions, actions and more importantly for this rant: THERE INACTION!
idk... maybe I'm overthink it but I don't think I am, let me know what you think.