You're right to point out the "relevance factor" is not what people commonly take it to be. The context is (as always) crucial. Of course, the degree to which an individual tolerates advertising varies for a multitude of reasons.

> billboards [...] countryside

I think people simply find this to be an ugly thing. They object to the ugliness of it. They're in the countryside -- i.e. not the town/city -- and they find themselves unable to escape (even here!) from this seedy miasma. Putting disgust into words is not a simple thing, perhaps this is the reason for the inconsistent reasoning you've noticed.

All advertising is ugly, it's an ugly business -- money grubbing manipulation. It's inherently weird to be subjected to the endless torrent of uncanny twisted art that is advertising every day for your entire life. The ads on Youtube are normalised by the same force that normalises all the other advertising -- the ads in one context normalise the ads in another. The ads on the side of the bus, on the LCD panels on the train, on the same screen that shows the timetable at the station, before the movie starts, by the seemingly sensible ads in Costco. One hand washes the other.