You missed the part about control. If they have the power, they should have the responsibility. And maybe the accountability as well.
A publishing platform can choose to be neutral (no power), or it can choose not to be. Of course safe harbour should apply to neutral media. For those that are not, however, I am not so sure. Almost, but not totally.
About the paper makers, you missed another point: anonymity. It should be very easy to reach the actual author of a paper, so this makes no sense to sue the paper maker. If the paper maker published all this stuff anonymously, and can't (or refuse to) disclose the author's name, however, that's a different story.
Which fits in nicely with what Google's done in this. They not only have a notice -> take down system in place, which removed the video within hours, but they even helped in finding the people who were the problem in the first place. They used their control to act responsibly. If they allowed it to stay on the site you could argue they're accountable for it, but its temporary existence is merely due to it being a user-submitted content host.
More-so than many other content-hosts, Google did act responsibly. If anything, Youtube is a medium just like the paper, but Youtube responds more rapidly and identifies criminals using the system uniquely, which is something paper cannot do.
Shall we eliminate all paper? It's easier to draw kiddie-porn on paper and post it up all over than it is to get away with it on Youtube.
Obviously, harboring criminals is different than helping an investigation to find the criminals. But without a takedown notice being ignored, there's no harboring, because there's no effective warrant. And without a legal takedown notice, there's no responsibility to remove anything, because to do otherwise would encourage people to restrict free speech just by complaining loudly.
Papermakers have the same measure of responsibility in this as Google. They can restrict selling paper to anyone until they see and approve the use of it, or stop selling altogether, or they can have a safe-harbor because they're merely creating a medium. If they sold paper to a book company that went on to write something which later got banned, they'd be in exactly the same position as Google, but nobody is saying we should all make paper ourselves, and affix our name and address to all the paper we make (running a server is easy to trace, telling who made paper isn't, so this is needed to level the playing field). And, as nobody can guarantee that that name and address are genuine on the paper, it cannot prevent anonymity, and loses significantly to a web host like Google who can point you to the source of everything they find.
Please, everyone, stop buying paper because people can abuse it. In fact, stop buying water because someone could use it to drown someone else, and the water company isn't doing anything to prevent this.
A publishing platform can choose to be neutral (no power), or it can choose not to be. Of course safe harbour should apply to neutral media. For those that are not, however, I am not so sure. Almost, but not totally.
About the paper makers, you missed another point: anonymity. It should be very easy to reach the actual author of a paper, so this makes no sense to sue the paper maker. If the paper maker published all this stuff anonymously, and can't (or refuse to) disclose the author's name, however, that's a different story.