The ad-tech network probably does make it easier, because any page that includes their ad network could deliver the malicious ad.
Streaming services used to be bad enough with ads in the shows, now they put an ad on the screen when you pause it. Peacock just started putting ads in the login screen.
If we don't stop this, pretty soon they're going to have ads in the stars as satellite constellations arrange themselves to sell you Twitter subscriptions. They're not going to stop even when there's an ad no matter where you look.
I'm reminded of the ending of the 2008 film Hancock in which Hancock defaced the moon with a corporate logo. But it was okay because it was "for a good cause".
Putting a static, printed poster on a wall is in no way analogous to surveillance.