Which is UK for Bitstamp itself, but Slovenia for their bank.
It's the US citizens that also have to abide by those laws when doing business with Bitstamp.
If US customers are specifically targeted by Bitstamp, it clearly is subject to US regulation (from an US point of view). Same thing if it only did business with US customers but was completely kept in Slovenian or something, thus not specifically targeting US citizens, US laws would still apply.
The only question is: "Can the US authorities excert their power over Bitstamp or not?". If Bitstamp has no legal representation in the US, it may not be liable to court orders or prosecution but may as well be shut down if the American authorities extend their influence across the Atlantic which is very narrow between the US and the UK, if you know what I mean.
This is how bitcoin-24.com got shut down btw. It targeted German customers but was incorporated in the UK as well. The banks didnt care.