So only citizens in the U.S?
nope
okay, heres basicly it
sopa was supposed to allow them to sieze sites OUTSIDE the USA, what they did to megaupload was what they wanted to do on a MUCH larger scale
megaupload had some servers in virginia, so they siezed it from there, 25% of americans use that file hoster, think about how much copyright material was on it? and if they have records on every single user who ever downloaded from there? no, doesen't matter where you downloaded it from, you used that site, they will sieze you in any country you are in, bring you to the USA and throw you in jail