What you’re overlooking is that most people don’t have any idea that this is going on at all. The shutdown, with the banner explaining the reasons, is a clue-by-four to wake them up.
If they don’t know about SOPA (which is pretty hard to do if you’re already hanging out on the internet) then finding their favorite site shutdown will just annoy them at the site, not the government. They won’t bother to research it, and even if they did bother to research everybody’s favorite place to start research (Wiki) is one of the sites going dark.
Good point.