They may have premium perks, but that doesn't mean they choose to use them. I dunno how it works, because I don't have a premium account, but from what little I've seen, they get more options, not necessarily different ad blocks.

It'd probably be a PITA just to customize all the different ad blocks for a site like CNN's - many are template pages, but they have lots of different templates and ad spots. I can barely keep up with the different ad blocks (and channels) for my own sites as it is.

I've never understood why some people are so gung-ho to get rid of the "Ads by Google". Even if I were big enough that it was an option, I would probably end up leaving it up there.

I have spoken to many "ordinary" people who have no clue how the whole ad system works. They said they never noticed the Ads by Google nor do they care. For them an ad is just a link and if it is exactly what they are looking for they click on it.

A lot of webmasters assume that people hate ads or will love to install those ad blocking toolbars. The reality is that people hate those intrusive popup ads or those banner ads flashing but in reality when people are ready to buy a digital camera,for example, they want to see ads from people who will sell it to them at a good price. If those ads are served by Google or not is not the point.