I'm not sure if I missed something, but as far as know the US Gov is not preventing anyone from participating in ICO's. As far as I know it's the potential threat if the SEC decides to hand down regulations retroactively.

The developers are just using proactive measures, such as confirming via a check box that you are not a USA citizen, just in case this happens.

Even if the US gov were to make it illegal there is nothing they can do to enforce it. As we all know the nature of a blockchain is such that anyone in the world can participate as long as they have an internet connection and the appropriate token.

Happy 4th of July!