The exchanges are actually on a weired situation, most of them are aware of the SEC and the different US regulations, remember what recently happened to BTC-E, even if it haven't started from the SEC at least, they reached them outside borders.
I believe this is why Bitfinex is now refusing US customers. There are more regulations every day, different countries are trying or starting to regulate bitcoin and cryptocurrencies markets, this is a serious limitation for exchange listing. Even the ethereum tokens are under regulation threat.