Without SOPA: Let's pretend that there is an innovative site that is going to be made in a few years that allows people to submit things to it and sell. This site gives some creative person a chance to make a full living. (this is actually happening now in many places)
With SOPA: Two options: Site either does not get created at all because of fear of creating sites that might cross SOPA at some point or this site gets taken fully off the internet because some other user on it posted some sort of infringing material for an indefinite period of time. in either case the creative person is out of luck with respect to making a living from their creative work.