Yes, I think that excluding americans citizen would be the best things! when people partecipate to the ICO would have to declare that he is not a USA citizen (and of everything other countries that cause problems)... This will put the MASS Corporation outside of any problems.... and if someone lie during the ICO participation and declare that he's not a USA citizen but he is USA citizen, well it's a problem of that person and not of MASS Corporation, because to partecipate he has done a false declaration, and MASS corporation is outside of any problems.
It's so simple!!
It's not that simple, legally. The SEC wouldn't go after individual investors but the company. And the SEC believes the long arm of the law is global and borderless if there's any U.S. citizen nexus in any way, shape or form.
Anyway, for MASS to do a legal ICO to U.S. citizens they will have to follow the regulations for Title II (Regulation CF), Title III (Regulation A+) or the older Regulation D. Title II and Title III require using an online crowdfunding portal to fund raise, but the ICO would be available to anyone in the world. Regulation D is for accredited investors only (and you can no longer self-affirm as in the past, the issuer has to actively verify).
Regulation CF is relatively cheap and easy to do but is limited to raising $1 million a year. Regulation A+ has two possibilities, $30 million or $50 million per year, but is relatively expensive on the order of about $100K due to increased compliance requirements. It is effectively a mini-IPO. You can do a CF/A+ and D simultaneously and there's certainly no reason this part couldn't theoretically be limited to U.S. citizens only, while the rest of the world is caveat emptor as normal.
Fortunately, StartEngine.com (founded by an Activision co-founder) plans to enable ICO token offerings for funding campaigns in a few weeks. None of the other portals have that in the works AFAIK. So that is the only option at this point for MASS if they want to include non-accredited investors.