Mainly out of sheer meddlesome lazy insensibility.
Basically, they think that people talking in public to anyone that wants to listen is the kind of thing that should be regulated. When I say, for example, that John McCain is contemptuous of those of different opinions and cowardly in his approach to commerce, and I suggest that defeating him is a good idea, and electing whoever runs against him, I am making a political statement available to anyone.
Well, folks, here's the thing. I am a citizen of this country and I can make any damn political statement of my opinions I want to. And you can disagree with me. That's the point. If you're offended, go ahead and tell me off. But don't expect that I will stop arguing. Anyone who wants to regulate what I write had better accept the idea that I will continue to write whatever I want.