Most ICO and IEO I had invested in always exclude USA and Canada from participating in their project for fear of SEC prosecution and eventually destroying their project. They have a law protecting their citizens against unregulated ICO, so having them as investors can spell doom when reported.
I have seen US citizens who invested in ICO, but most of them have dual citizenship. So, they just use other citizenship, which allows them to invest freely. there are also some of them that invest through their friends.
It is normal for every cryptocurrency project, most especially ICo and IEO to include it in their disclaimers that certain countries, such as the US are not allowed to invest, but if they do, nothing will be done against the project team.
This is why some projects prefer STO, but most of them in the space are illegal.