What's the causal connection between blog traffic and blog advertising?

Gordon Smith observes that all but three of the highest traffic blogs have advertising. What's the causal connection? I doubt that having ads makes you popular, though there is a slight chance seeing ads gives people the feeling the blog is popular/professional and makes them want to keep visiting. It's more likely that having high traffic makes you more likely to accept ads, because you can charge more for them.

And maybe the sort of person who writes a popular blog is also the sort of person who would both want to make money and accept the idea of ads on the blog. That is, the sort of person who would have a thing against ads also tends to be the sort of person who writes in a way that doesn't appeal to the crowd -- but I note that Gordon didn't study how many lower traffic blogs have ads. By the way, Gordon does not have ads.

Note: He's relying on the TTLB traffic ranking, which is limited to Site Metered blogs. He also notes the TTLB problem of blogs that share a Site Meter, making it look as though each blog gets much more traffic that it does. I note that if advertisers then rely on the Site Meter, they are being duped.