@Tamerlane probably something as simple as not wanting people she actually knows to know she works at a sexy maid cafe. Add in possible rules involving school and employment, and this becomes potent blackmail material.
@luvicvis those places aren't 18+ only. Minors can work there, wearing that kind of costumes is fine in Japan. The only thing minors can't do in those stores is drink alcohol. (Legally that is. Many of them still do though)
@Worf they can work there but they cannot be a hooters girl(wear the revealing clothing) until they're 18. 21 in some states. So yes that would illegal if it were hooters or any other "breasturants".