While I'm not a lawyer in the land of freedom (US of A), but I am qualified in my country of residence (therefore am not able to give this as legal advice so please note this is not that, just my quick thoughts). I've had a quick look and I wouldn't classify this as extortion. They are not asking for your property (the closest possible ground) they are asking for your personal information. This is really just a breach of contract issue. When you deposit money to Poloniex is is similar to depositing it with a bank, the money is no longer yours but the property of the bank. They essentially become a debtor to you and are answerable to repay the money.

So in short this is not criminal behavior, it is a civil dispute. You may be able to file for breach of contract in your state of residence but i would recommend reading over Poloniex's terms on conditions (always a good idea when someone is being a pain) to see if they have contractually set the state of contract as their home state, which i believe is Delaware.

Another thing to look at could be around any fair trading/consumer guarantees legislation and regulatory bodies that could slap them over the wrist.