I could give a lot of results-oriented arguments, of course. I could point to the wrecked and wasted lives of those who say something the censors disapprove of, and as a result are imprisoned, lose their jobs, lose their families, lose their ability to work. I could mention the thriving civic life that cannot coexist with the fear of saying something a law-enforcement official considers inappropriate. I could point out that taking the Internet, the first mass medium where anyone who wants to speak his piece can do so to people all over the world, and turning it into a glorified version of television would be a goddamn shame.
However, while these are all valid arguments, none of them touches on the basic moral issue. The answer is a pretty simple one, really: they shouldn't do it because it's wrong. It's a moral outrage to use force and the threat of force to stop people from saying things because you disagree with them or disapprove of them. And it's beneath a civilized people to answer words with violence.
We shouldn't do this to each other because it's wrong. For once, it's just as simple as that.