I want to ask, if I am accepted into College A from the waiting list. Can I decline this acceptance because I am accepted to another college? I know that this is bad because I kinda take the place of another student in the waiting list… But is this illegal?
No, there’s nothing wrong with turning away an offer from a waitlist. People do it all the time. However, if you don’t want to attend anymore, the polite thing to do is withdraw your application from the waitlist.
you’re fine. The wait list is a contingency for both parties – you agree to remain viable – they agree to possibly extend an offer later. If that offer arrives, you’re free to accept or decline. Indeed, even if you accept – and say another school’s WL offer arrives which you prefer, you can even later decline your first WL offer.
wow 
thank you guys so much