I don't understand how users own their own data. That doesn't make any sense. Where does that idea come from?

The same way that my bank does not own the contents of my savings account... Even if it is allowed to use that money (in highly limited and regulated ways) to, say, issue loans.

You don't own your users' data. Your users do. You may be allowed to use it in highly limited and regulated ways.

The legal concept is that despite having the data in your physical possession and control, you're not allowed to do whatever you want with it, and you have to ask the user's permission for many specific use cases.

This means that for a colloquial understanding of "owning data", you don't own it (since you can't do what you want) but they do (since they can limit the uses to what they want).

Photography laws are similar. I cannot just see you on the street, shove a camera in your face and take your portrait and then proceed to do whatever I want with the image. The resulting image is property of both the photographer and the subject. (Exceptions for people in the background of landscape/architecture etc and 'people of public interest' such as politicians.)

Common sense?