That is best known as SCAMMING, else the best approach would have been to give a deadline for those unwilling to verify theirs to withdraw and close the account. The policy wasn't there at the onset, so about the time they thought to introduce it, any good company would not settle to rob its clients and that exactly what Poloniex, Bittrex (both American companies) have done hiding under the cover of verification.
Well said, but I wouldn't limit it to USA, companies in my country does the same - and by "companies" I mean cryptocurrency exchanges, what's even "better" they hide this fact from users, I was trying to find anything about it on website of one of the exchanges and there was nothing except vague, broad mentions of "verification" in the terms & conditions, tried to contact them to find details and it took about a week to get info. Of course such behaviour (locking clients funds) is unacceptable and I think you would win in every court if you would take your case there, no matter what country (except, of course, if it's in country's law).