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.
Okay understood, thanks for the input! I'm just trying to figure out if as US citizen participated in an ICO with a VPN without any firm US laws stating that US citizens cannot participate whatsoever, why would I be possibly pursued legally? From my understanding the ICO providers do this to protect themselves legally. I understand the risk of being scammed and if I am that is at my cost.
it's not the authorities you'd have to worry about. who the hell's ever heard of some egyptian being jailed because they bought some shares in america behind a vpn?
it's the ico operators you should be concerned with. they'll all be so scared of triggering the americans they'd shut you down the moment they sniffed you out.