Ads have a place in the world, where we expect to see them (whether we like them or not), and typically most ads are not trying to pass as non-ads (yes of course there are exceptions to this).
The difference here is that these exist in a place where ads should not be, as per the description and use of the service. And it also subverts the experience the service owner is trying to provide.
Imagine if you accept a "free sample" box of cereal and you get home and open it and it's just full of flyers, instead of being full of cereal.
Or this is why you can't just go to any private space like a shopping mall with a megaphone and a sandwich board and start advertising your services without permission. Security will ask you to leave, because the owner of the mall didn't agree to this.