Unless I'm mistaken, the citizen wouldn't have violated anything as the onus and restriction is on the exchanges.
Crypto-startups not including US citizens in their ICOs is a precaution for any possible regulation that may eventuate - and don't forget many of these are truly global startups where, for example, something legal in Sweden may not be legal in the US - but I have never read anything that has made me feel that US citizens (and no, I'm not one) have anything to fear from the SEC. Yet.
As far as I understand it, as long as you're including any gains on your tax return you are obeying the law as it stands in 2017.