Wait, I thought .com/.net (among others) were generic TLDs governed by ICANN? I would assume .us might fall into this warning (as would other country code TLDs), but .com and .net?
Not exactly fair, but that's the current 'system'..
I don't know that much about how many are or aren't operated out of the US, but most of the generic TLD's are, and several ccTLD's definitively are too - for example .tv and .cc are operated by Verisign, and .co is operated by Neustar.
One of the more controversial parts of SOPA is the ability of ICE/Customs to be able to "un-resolve" domain names under other TLDs and country codes. Since the USA does not have jurisdiction over Russia (as an example), there is no current way to block a filesharing or spammer operating with a .ru address without having a partnership with Russia. This aspect of SOPA would allow the USA Government to block all domain name servers from resolving that address properly, by basically poisoning the legitimate DNS entry with one that resolves to some landing page operated by ICE. They won't be able to seize the domain, but they'll make it just as worthless to anyone inside the USA.