Sure, they control where the ad is displayed and how big it is but I am pretty sure they are not picking and choosing the ads. The space is probably sold as a service to an ad company, like how WoWhead and the current cursed forge web site do. So while they need to make sure they do a good job vetting who they rent the space out to, I doubt they vet every ad that will be there.
I don’t remember my old post. Btw I don’t use Firefox. Didn’t know it was still around! 
I’m sure if you’ve ever made an addon before, but it’s easy and cost nothing but time and authors would be bending over backwards to be hosted on a legit Blizzard server
Always knew you were a marxist. Surprised I missed this comment.
I recently heard that you can pay them like $3 a month to turn off ads?
Not yet since ads aren’t active yet but yes ad free will be $2.99 per month.
You are only looking at one aspect of what it would require. The majority of the cost would be in the server to house the repo and the traffic it would use to DL mods to us. Plus they wouldn’t need a review and approval team for mod uploads and updates. The majority of the cost of running such a server is not in the mod authors but the hosting service itself.