If you were to exclude non-USA companies you would be excluding some pretty large players that are currently operating out of the BVI, Antilles, Gibraltar and all the other offshores.

For example, look at the whois on GWWs biggest sponsor:
http://whois.domaintools.com/topbucks.com
Net Transactions Limited
Net Transactions Limited
C/O Net Transactions Limited Chuchubiweg 17
Curacao, AN
AN

These days, its almost more dangerous to promote US sponsors LOL!!!!