US citizens are not allowed to participate.
It is because if you accept American customers, you accept American jurisdiction over your entire organization, which means that if you fail to follow their regulations you have the possibility to be in prison. This is why some ICOs doesn't accept US citizen due to this rule that they are implementing.
Even so? Possibility to be in prison? That's a serious reason. Now it is clear why almost all ICOs forbid US citizens to participate.
I wonder if China has the same rules? Usually in ICO among the forbidden countries are the USA, China and Singapore. By the way, here only U.S. citizens are not allowed to participate, or is there someone else?