One other excuse that is invoked is the protection of children. It's really pointless to attempt to protect a child from himself, especially in matters of natural curiosity. One of them, of course, is to be popular. The government sways with the wind. If the people want censorship, it responds in kind, regardless of how in so doing it attacks our right of free speech, and ultimately, of property.
Information is power.
That sounds like a trite aphorism, but it's true. If one controls the flow of information, one basically controls the population, for information is awareness. If the government takes away the information from the people, it takes control. That's why censorship is the first step to tyranny. A tyranny must always, like any idea that cannot stand the light of reality, eliminate any compromising information.