that's the way they say:

"hello guys
we stole from all of you your btc, this is not enough, and for that come one again to make for us some money!"

and who says that you can't sue them?
the site is running by him self? the owner is god?
i don't think so.
i think that are 3 ways of solving that.
1 the simple one, just pay the debt
2 the legal one, where they can lose a lot more money
3 everyone knows the third