Using it to sell you shampoo isn’t terrible (it can be super annoying though). The problem is using that data to eg figure out who might be in the market for pregnancy related products. Or, ominously, who have stopped buying pregnancy related products early.
Or correlating interest in something they browse with voting intentions. Or interest in political action. There’s a lot of dodgy things you can do with that data. And little of this is being shared with *informed consent*.
Ads are NOT the problem. I love browsing through ads in magazines I buy. If online ads worked like outdoor ads or magazine ads, I suspect a lot less people would have a problem.