IIRC, you pay them (not a lot) to check out your ads and verify that they are, in fact, unobtrusive. I realize it's distasteful that money changes hands at all, but I can imagine it takes a non-trivial amount of person-hours to vet these things on a large scale, and it should be easy enough to tell if they're crooked or not just by using ABP and seeing if the permitted ads are intrusive.
When I used it (until after their ad whitelisting came in), I found that they did a poor job of blocking obtrusive ads, then. Either that, or their standards for "unobtrusive" were low.
I switched to a better adblocker, so now I just whitelist sites whose ads I'm willing to see.
