Because nothing says that something is unquestionably true more than banning all criticism or questioning of it. Apparently they think if they silence all dissent, that means they're proved right. Anyone with half a brain can see that's exactly the opposite of the behavior by someone who's telling the truth-- the truth does not fear being brought under scrutiny. What they're trying to do is prevent our means of communicating and getting new people to question the system. If only their propaganda is allowed, no one will leave their bubble-- or so they think. I think, and hope, that this causes a massive backlash since more and more people are not trusting their bubble to tell the truth anymore.