To be honest, I think this is the reason Google doesn't have this feature. The sites everyone wants to blacklist are the spammers that game Google search and show Google ads. If they don't get traffic, they don't show ads. If they don't show ads, Google doesn't get that money either.

It's to Google's benefit that people end up on these pages, see a ton of ads, and then click on one out of confusion or desperation.

That's strange, though, because it should be clear to them that the long term effect of this is pretty dangerous for them - if their search results were always the best, then they would be largely unassailable in the search market, but as it is, they're decreasing satisfaction with their product. It's hard to improve everyone's perception of your product once that's happened, and it opens them up to usurpers.

It does, but Microsoft's thrown loads of money at the problem and not made huge inroads so maybe they figure it's worth the risk?

Yes, but that would be pretty evil of them, don't you think? ;)