It's really not their fault, what you can really do it to verify your account in order to prove that you are not a citizen from U.S. , Bittrex might be legal in other countries but not in U.S. It is not about stealing your money it is about maintaining their business  as a cryptocurrency exchange and if they experienced issues like this they might faced a complaint for their business and the worst scenario they will be shut down with all the other funds of verified users. It is really good that this kinds of businesses are complying with legal requirements.