So, in that case, the FBI could seize any domain as long as the operator is within US jurisdiction?
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.