I don't think anyone is saying they're fine with it, but they all know about it. They know what they're buying, and they see the ratings for all of the spots they buy. So they know if people tune out during long stop sets. Here's what I tell all advertisers. They can be the ONLY advertiser in a stop set. They can ask to be the first spot. We have a price for that. Nobody has ever taken me up on that. What some have done is buy two :15s, one for the beginning of the stop set, and one for the end. So they know about it, and they've addressed it.
The OP's comment assumes people turn the radio on, and listen non-stop. What we see is they don't listen that way. They start and stop a lot. Most of it has to do with the fact that they're listening in the car. They get where they're going, and they stop. They get back in the car, and the radio turns on. It may be near the end of a commercial break. In that case, they didn't hear the first spot. They heard the one at the end. Or perhaps they tuned away, and came back. Nielsen documents listener behavior, so the station knows how many people tune out because of commercials or a song they don't like.
You're wrong. They know what they're buying. As I said, nobody buys one commercial. They buy hundreds, sometimes even more. One spot in one long break means nothing in the context of the overall buy. You have to understand they're the reason why you hear so many commercials, and why they repeat so often. They not only know what they're buying, but they create a lot of the problems listeners have with spot breaks. The repetition, the quality of the production, and even how loud they are. That's done by the advertiser. They want to create an impression. Even a bad impression works for them.