So that they can effectively do the same to sites not based in the U.S., like MegaUpload was. Obviously they will never be able to shut them down, but they can block them from us. I love how the Constitution, the basis for all our laws doesn't seem to be in the equation to these politicians, which it is clearly against censorship.
Even worse, these politicians forget who they work for. They are public servants, and are SUPPOSED to be the voice of the people they represent. Instead they choose to either follow their party, campaign contributors, or in this case companies with lots of money, which I can make the safe assumption that they are getting something out of it personally. It seems that personal agenda is the only thing they choose to follow, and they people they represent, who "hired" them, are just afterthoughts.
This a spreading problem in the U.S., unfortunately. I am happy to see that at least 35 (last I heard) senators publicly denounce the bill. It will only take 40 to have the bill killed in the Senate, so that is pretty close.