Having any ICO for US residents is a tedious work at this moment. Lots of government agencies will be behind you to know the details of the investments you are getting and how much you really want to create a project.
There were some icos was accepting US residents last year. But nowadays didn't see any legit ico accepting them. So even dusk not a security token, i can easily understand why don't accept US residents like all other projects.
Yes, the reach of the US government goes far. If they are against you your project is dead so it is better to avoid the US altogether.