That's the best situation, since they know the content and can segment their content very effectively. YouTube Partners and VEVO can get premium pre-roll ads because it's considered to be premium content. Content that gets flagged or has negative keywords in the title, desc, or comments may get no ads at all.

Any publisher can do this on their own. They can sell their own ad space at a premium, and use an ad network throughout the rest of the site -- it's called "remnant" inventory. If someone writes profanity or uploads something for that page, them switch the ads off, or use a different ad network that will tolerate that. High traffic sites frequently use multiple ad networks to maximize revenue yield.