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.
Yes, the case is difficult. What Poloniex has done is illegal in the US, but the exchange says we can do it because we're not offering this service to US citizens. It's a bit like saying drugs are illegal, but there would be nothing wrong selling drugs to a Chinese tourist...
There's also lots of regulation for consumer and investor protection in the EU, but they would not apply to investments outside the EU.
Poloniex is based in Boston, so justice has to be there.