You're technically correct in that there isn't a US legal ban on its citizens. The exclusion of US citizens from most ICOs (or any other crypto service offering investment vehicles for that matter) are pre-emptive of regulatory measures most people feel must happen soon. It's just cautionary so they cover their asses legally down the line, so to speak.

Personally, I doubt you have much to worry about if you're thinking of using a bypass (VPN, for example) for short-term participation. For now at least, in practice, it would be very difficult for authorities to pursue you anyway if you also cover your ass in privacy-wise.