Say your neighbor is running a meth lab in his basement. In order to combat this, the government passes a new law that no meth houses can have street numbers. A police officer comes by and removes the numbers from your neighbor's house.
Does the meth lab still pose a risk to your neighborhood? Has the law successfully addressed the flow of meth into your community?
This is how SOPA works. The government takes away a server's street number (domain name). The server is still there, and anyone who knows where to find it can still find it. If it happens enough, then the people just come up with their own address system and start ignoring the one that the government controls.
That's pretty straight forward isn't it?