It's a little dicer in the cases where we can pretty much infer that they're trying to get to a specific thing and would have happily used a direct URL to the page if they had the wherewithal to do so. In this case the ad placement is basically a brand trying to hijack my attention while I'm in the process of seeking a thing out.
I struggle to think of a meat world analogy. It would be something like if I dialed my girlfriend up on the phone and, instead of routing me straight to her, I had to navigate through a switchboard asking me "How about talking to these sexy singles in your area instead?" And in order to prevent this, my girlfriend would then have to pay the company to route my call straight to her.
Of course this isn't a perfect metaphor because there's a lot of different ways people use a search bar, especially now that search bars are merged into URL bars. But that sort of gets at what it is about this that feels sleazy.
