They're creepy as fuck. Same reason someone might not like being followed around everywhere, even if the person following them just writes down stuff about them but never does anything else. Spying ads are like that, but worse: it's like that person also occasionally runs in front of you to slap an ad-bearing sticker on some surface you're about to encounter, based on stuff they've written down.

Spyvertising is that, but at an industrial scale. If one's creepy and ought to warrant intervention by law enforcement, the other's much, much worse.