I personally don't really see the problem. I'm not going to buy something I don't want/need. If they use the data to show me ads for something I want that then leads me to making a purchase, to me that seems like a win/win.

I'm generally a pretty frugal person, I have a good handle on my personal finances and I don't really buy much stuff at all. Maybe for someone else, shopping can be similar to an addiction where they see an ad and they know they shouldn't buy it but they can't resist. Women with a closet full of shoes come to mind. For those people I can see how this might be problematic but personally I largely ignore ads altogether. If I need a small thing I just go to the nearest store and buy the first best thing I find, if I'm making a more significant purchase I generally do some research and look at tests, user reviews etc. Sure there's some opportunity for manipulating me with fake/biased tests and reviews etc but I'm not really seeing how my spending info helps with that.