I don't think it's illegal for citizens, more like it's illegal for icos to sell to US citizens who not accredited investors.
https://www.sec.gov/news/press-release/2017-131"The Report confirms that issuers of distributed ledger or blockchain technology-based securities must register offers and sales of such securities unless a valid exemption applies.
Those participating in unregistered offerings also may be liable for violations of the securities laws"