I think it's pretty murky, ignoring a subpoena is a crime, so US may be able to charge them with obstruction and request extradition, it's then on their home countries to decide whether to accept the US's requests. Either way I'm sure it would make them ever traveling to the US very tense.

See: Julian Assange

Remember though, these instance admins are generally doing this out of the kindness of their heart on shoestring budgets, it's so much safer and easier for them to just comply with legal requests. They're nice people, but not political martyrs.