The problem is that it leaks information. If I place an ad for paperclips targeted at cat-lovers, then if you click on my ad and buy paperclips, I know that you are a cat-lover.
I'm not sure I understand your point. It only leaks a subset of data (cat lovers who have an intent to buy paperclips) and is hardly valuable at scale to anyone for anything other than prospecting new customers.
I dunno, identifying groups like "Jew-Haters" seems useful beyond "prospecting new customers." For example, if the US government cared about tracking hate groups (as opposed to hiring them into the administration) they could use it to identify people to track.