Oh, I forgot to mention, there's also regulations around "commodity derivatives" if you allow the purchase of such products by US customers. An example of a derivative would be the bitcoin future contracts offered by BitMex that allow you to invest a small amount in a "leveraged" manner to invest at a higher risk level for a potentially higher return on your investment. In order to offer such investment opportunities to US customers, you would have to get them approved by CFTC (another US regulatory body, this one in charge of commodity derivatives). The CFTC fined Bitfinex ( a Hong Kong-based company) a while back, and this was probably the initial catalyst for Bitfinex to drop US customers.