There is language in their TOS that I believe relives Poloniex from legal responsibility from covering the losses.
terms are not enforceable in court where they contradict the law. so the question of jurisdiction is crucial here. like countryfree said, haircutting balances with a generalized loss is 100% illegal in the USA, where poloniex is organized. but IANAL and i dunno if they've dodged a bullet by restricting USA customers from this market. if so, what laws apply here? EU? various local laws where lenders reside?
i wouldn't be surprised if the EU offered similar protections to the USA but i have no idea where to start researching the law.
I am curious to know why you believe "haircutting balances with a generalized loss" is illegal in the US. I think this is exactly what happens to depositholders at banks whose deposits exceed FDIC insurance limits when they fail.
I would also point out that I do not know if Poloniex has shareholder equity equal to or greater than the amount of the loan losses. If equity is less than the amount of the loan losses, suing Poloniex may only result in you paying a lot of money in attorney fees, and Poloniex declaring bankruptcy.