Many European ICOs recently that I saw, they all state in their ToS that the US citizens can't participate. Why?
It's not so, you misunderstood everything.
indeed, in a bounty program, it is sometimes explained that some countries are not allowed to join either the bounty program or the ICO, but statements like this are not the US government which prohibits its citizens from joining cryptocurrency investment, even if it is only strictly regulated in the rules. this investment as explained by the friend above