So magic is considered sacred and something only witches can use. This is believed by all of normal society (non-witches). However, it is not. Anyone can use magic by simply drawing specific symbols a certain way.
This is a HUGE secret that every witch is sworn to secrecy on. If word gets out this kind of power could very easily fall into the wrong hands and things can get really bad really quickly. (for example there's an arc that happens in the manga that shows just how fast things can escalate and how devastating the effects are. Showcasing exactly WHY this precaution is implemented.)
In order for this secrete to remain secrete, memory eraser is allowed. There are three different degrees of said spell but the one commonly used is the 1st one which only removes a subject’s memory of a recent event. These usually do not erase more than half a day at most and some can even do just a few minutes.
In all the use of memory spells are seen as “an exception for the greater good.” If you have any more questions I’d be happy to answer!!