if they could prove you were american you'd be kicked out of whatever the distribution was and your money returned.
they're gonna be more than a little more concerned with staying out of legal hell than keeping one american customer happy.
With the amount of US citizens participating in these ICOs, I really doubt the provider would waste their time and resources hunting down thousands of individuals who may have participated in their ICO. Especially with how anonymous the crypto market can be. The blanket approach is to just state you cannot participate in the ICO from the get-go, which the majority do anyways. They definitely know people will use VPNs to bypass this restriction. That being said, it is still a legal gray area. No law has been passed blatantly restricting US Citizen participation.
It's no different than exchanges that prohibit US customers, but don't require KYC. You can be damn sure that US customers will still be using Bitfinex 3 months from now (albeit through VPNs), just like they are using Bitmex and Okcoin through VPNs right now.
This is the next phase as we move towards regulation. Service providers and, in this case, securities issuers, need to make a good faith effort not to run afoul of the US government. For now, it seems that asking users to confirm they aren't US residents and maybe banning US IP addresses is enough.