It's very simple. The SEC will prosecute one of the big boy "exchanges" and bring that company to a screeching halt. All of the rest will have to meet stringent regulatory requirements in order to service American customers.
It's obvious the SEC intends to enforce securities regulations on crypto markets. The big exchanges already and typically employ experts in regulatory compliance specifically related to AML and KYC laws on a federal and state-by-state level. The reason being access to Americans is by and large more important than access to any other set of customers. And "everyone" knows ICO's exist only to side-step securities laws and make a quick buck. There are very few legitimate offerings. Expect the SEC enforcement arm to come down like the hammer of god.
What I find funny is this idea crypto won't be the next level of monitoring and control of the money supply. You crypto proponents are going to be very unhappy when you see what Uncle Sam has in store. Fiat currency has much better built-in anonymity then this block chain nonsense. And see how that is controlled, regulated, and tracked through treaties, laws, and digital mechanisms?