What if a user uploads something to an advertising supported site?

So I was reading the Oatmeal the other day, and ran across this nice blog post.

In it, our hero describes how some random internet site has been "stealing" his comics without credit on their for-profit website.  To be precise, he says they were "stolen, re-hosted, and monetized", which you know is something I'm definitely not in favor of (although I might take issue with the "stolen" characterization).

To add insult to injury, the loser is actually demanding money from our hero, the actual (and often brilliant) creator of the comics, for "defamation" for claiming that the rogue site still has his comics online.

The site operator's defense is that they did not steal the comics, but they were uploaded by users.

Which brings up an interesting question.  How does this fit into my ethos?

It is very much like the Megaupload situation.  Megaupload has been attacked by the U.S. Justice department for the content that its users uploaded.

This is particularly ironic in the case of Megaupload, because it appears to have been a destination site for many Hip Hop and R&B artists whose music was regularly "pirated" there.

Now this says nothing about the obvious asinine drivel which is the request for $20,000 from Matthew Inman, the creator of the Oatmeal.  He's shown quite a bit of restraint, even now, I say.

But what of user submitted content as a matter of principle?  I've talked before about the practical impossibility of creating an automated "content filtering system".  Should not a person be able to create website and allow users to upload content?

Since it's impossible to filter all that content, should a person be allowed to monetize the site, by my fair use lights, even if users are uploading copyrighted content to it?

To be honest, for once I do not have a ready answer.  I suppose it might be practical to say, ok, you can put ads only on pages where you control the content that is there.  For example, the main pages of the site.  But forums or other areas where users can post things must be kept clear?  One could make a case that the content is still being monetized, of course, but I do think you have to draw the line somewhere.