When Asterix and Obelix join the theatre group, the group's entire performance consists of them making faces, lampooning "proper" theatre, being generally anarchic and insulting the audience, and the prefect sits through all this with a bland smile on his face, evidently enjoying it. When Obelix gets his moment, and all he can think of to say is "These Romans are crazy", the prefect is immediately enraged and orders the entire troupe to be arrested. Why didn't he do this earlier? Because Asterix and Obelix aren't Romans. The prefect is fine with Romans mocking Rome, but when foreigners do it, it's a crime.
