she is technically doing it to protect the princess, who was already targeted, though.
and she is a superpowered hero from ancient times and the power to match if she puts her mind to it, apparently.
I take it not so much as letting it pass because she is a cute clumsy girl, but because she is too much of a danger to imprison without enough measures that some big justifications are needed, and while in the academy she is both just an overzealous but efficient bodyguard and under proper watch herself.