Our rules and intuitions about free speech and censorship are based on the types of censorship we usually see in practice. Ordinarily, if someone is trying to censor a piece of information, then that information falls into one of two categories: either it's information that would weaken them politically, by making others less likely to support them and more likely to support their opponents, or it's information that would enable people to do something that they don't want done.

People often try to censor information that makes people less likely to support them, and more likely to support their opponents. For example, many governments try to censor embarrassing facts ("the Purple Party takes bribes and kicks puppies!"), the fact that opposition exists ("the Pink Party will stop the puppy-kicking!") and its strength ("you can join the Pink Party, there are 10^4 of us already!"), and organization of opposition ("the Pink Party rally is tomorrow!"). This is most obvious with political parties, but it happens anywhere people feel like there are "sides" - with religions (censorship of "blasphemy") and with public policies (censoring climate change studies, reports from the Iraq and Afghan wars). Allowing censorship in this category is bad because it enables corruption, and leaves less-worthy groups in charge.

The second common instance of censorship is encouragement and instructions for doing things that certain people don't want done. Examples include cryptography, how to break DRM, pornography, and bomb-making recipes. Banning these is bad if the capability is suppressed for a bad reason (cryptography enables dissent), if it's entangled with other things (general-purpose chemistry applies to explosives), or if it requires infrastructure that can also be used for the first type of censorship (porn filters have been caught blocking politicians' campaign sites).

These two cases cover 99.99% of the things we call "censorship", and within these two categories, censorship is definitely bad, and usually worth opposing. It is normally safe to assume that if something is being censored, it is for one of these two reasons. There are gray areas - slander (when the speaker knows he's lying and has malicious intent), and bomb-making recipes (when they're advertised as such and not general-purpose chemistry), for example - but the law has the exceptions mapped out pretty accurately. (Slander gets you sued, bomb-making recipes get you surveilled.) This makes a solid foundation for the principle that censorship should be opposed.

However, that principle and the analysis supporting it apply only to censorship that falls within these two domains. When things fall outside these categories, we usually don't call them censorship; for example, there is a widespread conspiracy among email and web site administrators to suppress ads for Viagra, but we don't call that censorship, even though it meets every aspect of the definition except motive. If you happen to find a weird instance of censorship which doesn't fall into either category, then you have to start over and derive an answer to whether censorship in that particular case is good or bad, from scratch, without resorting to generalities about censorship-in-general. Some of the arguments may still apply - for example, building a censorship-technology infrastructure is bad even if it's only meant to be used on spam - but not all of them, and not with the same force.

If the usual arguments against censorship don't apply, and we're trying to figure out whether to censor it, the next two things to test are whether it's true, and whether an informed reader would want to see it. If both of these conditions hold, then it should not be censored. However, if either condition fails to hold, then it's okay to censor.

Either the forbidden post is false, in which case it does not deserve protection because it's false, or it's true, in which case it should be censored because no informed person should want to see it. In either case, people spreading it are doing a bad thing.