I use Spotify Free, i.e. with ads and what I find interesting that most (paid) ads are from Universal Music. So they pay for ads so you listen to their content (targets are playlist with crap pop music). Weird.
It's not unreasonable to assume that other labels (like Universal) have a similar deal, so those adverts you're seeing may well be unsold ad inventory that has been given to Universal to promote their own artists.
So, I'd question your assumption that Universal is paying for those ads.
1: http://www.theverge.com/2015/5/19/8621581/sony-music-spotify...
They could of course resell the ad space to external advertisers but may prefer to increase the exposure of their artists.