It seems like he's against the notion of "I disagree with what you're saying, but I will defend to the death your right to be misinformed."
Basically, if something can be proved as false information, and yet it's still allowed to spread to millions of people that unlike the average HN commenter will not do the research and end up believing the misinformation, what's the value in that? Why would we allow something that's false to spread?
This is a dilemma that creates a lot of dissonance in my mind. On one hand I support encryption, and believe that banning certain types of information on the basis of being "wrong" is a slippery slope when all of that hinges on what "wrong" means and who controls the definition of "wrong." On the other hand this kind of misinformation can cause material harm to society and innocent people who are affected by the misinformed. Without censorship or any way of shutting down this information before it reaches people who would hypothetically always believe it if they were to know of its existence, it feels like the most we can do is stand by and watch it spread. Imagine trying to convince millions of people to change their minds once they've been set for good, and that those people are now convincing your friends not to vaccinate their children, or similar.
I think what Gates is saying with regards to child pornography is that it's the only type of digital information that is prohibited to be in possession of by law. The reasoning behind this is that having child pornography distributable creates a market by which other child pornographers will have demand to keep producing more of it, which by the nature of child pornography means exploiting children. So I guess what is meant is that if you can be arrested for consuming one type of materially harmful information that provides no net value to society, why not stop other kinds of materially harmful and valueless information from spreading? There is already a status quo. Child pornography proves that there is a class of information that is so damaging to society to have publicly available that governments believe it must be outlawed (censored?) entirely.
However, child pornography different from publicly available viral videos. By its nature child exploitation doesn't spread virally online. It isn't mainstream. Viral videos on YouTube are specifically produced to reach as many people as possible, and being safe enough to exist on YouTube is pretty much necessary for this to occur. People would have to take steps to prevent child pornography from spreading to the wrong people, because of the nature of modern society where any average person would be inclined to get them arrested if they knew they were in possession of it. It is firmly ingrained in a vast majority of society that child pornography and exploitation is evil and the miniscule amount of people who believe otherwise will be endlessly shamed for their viewpoint and the shamers will never be debated on that point, because nobody generally debates things related to child pornography, and if you are discovered to like such a thing then you instantly become the scum of the Earth in almost everyone's mind who knows this fact. But those same people are happy to let harmful misinformation slide if they can't or won't understand what it actually means, information that could harm children just the same, like anti-vaccination material, but are instead marketed from the standpoint of protecting children instead of exploiting them. And if criminals are going to use encryption to hide it all, then that's how it's going to be. Encryption is just a tool. Criminals can buy knives, too.
Still, I think if we're going to allow misinformation to spread on the principle that all information should be allowed to spread, because of the fact that it's information, then we should be prepared to clean up the aftermath of the rioting later.