But then there are things that I don't want ad companies to know about. My medical history, my likely voting patterns, my political affiliations, my sexual orientation, the nature of my relationships with other people, etc. These are private, and I don't want ad companies (or anyone) to know these. Depending on the topic and where I live, it may even be dangerous to me for others to know these things.
One thing that has been made apparent by the advancements of ad-tech's excellent ability to find unintuitive patterns in consumer behaviour, is that the benign data can be used to predict the non-benign. So even if data collection is regulated to only collect benign data, or I am extra careful with where my sensitive data goes, I still have a problem.
That's why tracking on this scale is bad. That's why I hope we can build a society where we stop these practices.
When Nazis invaded a city, first thing they'd have done was getting to people register and getting names and addresses of "undesirables".
People have not learned their lesson.
Even that is a problem for me. Advertising is manipulation, they want to change my behaviour so I purchase whatever product they are selling. So I've gone from a state of not thinking about buying something to reaching into my wallet.
I don't want any corporation to do that to me which is why I'm against advertising in general.
Evil regimes have never had a problem finding lots and lots of citizens to kill in the 99% of human history before the internet.
Also things don't have to be as extreme as literally killing all members of a minority group for this to be deemed "bad". It can be as simple as targetted influence campaigns to push certain policies/agenda. The ability to influence on mass scales has never been easier and cheaper. There are many examples throughout the world of how that influence has been used. And while yes influence campaigns have always existed in some form, the degree of targeting and the ease at which this has been made is a case where and difference in scale is a difference in kind. This is a powerful tool that I don't believe anyone should have access to. States, companies, or individuals
That's a more extreme example, but there are lot of other ways creative government could make the lives of people they don't like miserable or impossible.
Advertising in Inc vs Wall St Journal vs People magazine vs Wired vs TV Guide vs Car & Driver vs Cosmo vs Ebony all gave you easy ways to target different audiences. It's more targeted now, but I don't think it's multiple orders of magnitude more powerful (mostly because the reach isn't nearly the entire story; you still have to influence after reaching.)
Still, I think it's clear that data can very quickly go from harmless to harmful depending on who gets their hands on it. The Nazis absolutely did have a problem finding all the Jews they wanted to kill, and abundantly available data about the religious preferences of literally every citizen being immediately available to them would absolutely have caused much more death than already happened.