I didn't even realise that so many people hate Tartah for joining the brimmed caps. I personally saw it as the only logical way for his story to continue.
If he had refused the offer, that would've been very out of character for him.
Think of it like this: You're disabled and can barely do this thing you're really passionate about. You struggle 10 times more than the average person, and most of the people around you tell you that there is no way to make this thing you're into more accessible. You're angry because the adults refuse to even look for ways to help you since it's illegal for some reason. Why is it illegal? Why is no one explaining it in more depth other than 'forbidden magic bad'?
Why wouldn't you turn to the only group of people who claim to have a way of helping you? You barely even know anything about these people. Are they really as bad as everyone says?
Can we, as the readers, even consider the brimcaps the bad guys? Despite their mysterious and creepy nature, they do have a point when it comes to making magic more accessible, even if it is some sort of manipulation tactic.
The pointycaps aren't that innocent either. Especially in Tartah's case. From his perspective, they are definitely way more suspicious than the brimcaps. Both groups are morally grey if anything.
And let's also note all the foreshadowing. From the moment we meet Tartah, up until the moment he joins the brims, we see that he's very frustrated with the system that the pointyhats have created. He constantly questions things. It is only natural that he turns to the other side when he gets no answers.
I also see many people point out that Tartah is a child and that he doesn't really understand what he's doing. I'm not a big fan of this take since I really doubt that he would've done things differently if he were an adult. Joining the brims wasn't some tantrum to prove people wrong, but a decision he made in search of the truth.
I find all the hate Tartah gets very sad. As a disabled person, I find his anger very relatable. Not every disabled person is going to do nothing. Some can and will try to fight against the system and Tartah being that somebody means everything to me.