They won't stop at US borders. They'll just use it as an argument in other countries. Eventually the Congress would be convinced to pass laws to punish foreign nationals -- deny visas, seize domains on US DNS servers and so on.
I can't think of a better way to erode Americas international influence. If you can't go there and your web sites can't be viewed there, and they won't let you watch any of their popular media, or even import your products for sale there, what the hell good are they to anyone?
I expect the makers of SOPA et al think they're dictating terms to the world. I think will turn out to be just isolationism in disguise.