I think Bitfunder made the right move by not shutting down. There is still a lot of trading to be done in the non-US market.
The fact that Ukyo is keeping the exchange open demonstrates that either he hired the worst securities lawyers in the U.S., or more likely, he is completely full of shit.
If he is in the U.S. he must register as a broker / dealer to operate Bitfunder, whether he allows U.S. residents to participate or not.
Source:
http://www.sec.gov/divisions/marketreg/bdguide.htmForeign Broker-Dealer Exemption (Rule 15a-6)
The SEC generally uses a territorial approach in applying registration requirements to the international operations of broker-dealers. Under this approach,
all broker-dealers physically operating within the United States that induce or attempt to induce securities transactions must register with the SEC, even if their activities are directed only to foreign investors outside of the United States. In addition, foreign broker-dealers that, from outside of the United States, induce or attempt to induce securities transactions by any person in the United States, or that use the means or instrumentalities of interstate commerce of the United States for this purpose, also must register. This includes the use of the internet to offer securities, solicit securities transactions, or advertise investment services to U.S. persons. See Securities Exchange Act Release No. 39779 (March 23, 1998)
http://www.sec.gov/rules/interp/33-7516.htm