How can U.S. regulators (CFTC) enforce that an offshore Fund not accept any U.S. clients? Isn't this beyond their jurisdiction? Is there an example where the CFTC has gone after an entity located outside of the US?
I don't know of any CFTC example, however there is a thread where Suretrader (a Bahamas regulated broker/dealer) was required to notify the IRS of their U.S. based clients. You can read about it here:
http://www.elitetrader.com/et/index.php?threads/be-careful-if-you-are-at-suretrader.288152/
This is just an example of more intrusion of private citizens. Apple, Cisco, Qualcom, etc. can have BILLIONS in offshore cash through foreign subsidiary corporations that are not legally subject to IRS tax, however if you dare open an individual 10k trading account with Suretrader, then you better report it "or else" subject yourself to the potential consequences.
Alibaba, for example, the giant Chinese B2B e-commerce firm, created a Grand Cayman Islands company (Alibaba Group Holdings Limited) for the sole purpose of listing its publicly traded shares in the U.S., go figure.
Of course, there is always a workaround even for an individual, such as creating an offshore entity to trade with an offshore firm if you want to keep your profits offshore. However, that requires a lot of paperwork, knowledge of offshore rules, a competent attorney and lots of upfront cash, which is not realistic for a daytrader with a small account.