The problem with banning things is actually a bit more subtle than just that. Propaganda in isolation isn't necessarily a good or a bad thing. If Big Vegetable embarks on a multi-billion dollar campaign to push pro-vegetable propaganda, that is probably a win for the rest of us. But if Big Tobacco does something similar it is more of a loss.

So we need a mechanism to decide if a specific type of propaganda is acceptable or not. And, if governments get involved, typically they will start restricting propaganda that is truthful but threatening to their chances of holding power (ie, people pointing out the flawed nature of the powers that be). It is better force people to fight propaganda with counter-propaganda and let individuals make their own mistakes.