You were playing a game without paying for it. How did you imagine they were making money without pimping your data?
Unsarcastically, a lot of people believe user data belongs to users, and that they should have a say in how it's used. Here, I think the point is that Niantic decided they could use the data this way and weren't transparent about it until it was already done. I'm sure I would be in the minority, but I would never have played - or never have done certain things like the research tasks - had I known I was training an AI model.
I'm sure the Po:Go EULA that no one reads has blanket grants saying "you agree that we can do whatever we want," so I can't complain too hard, but still disappointed I spent any time in that game.
I can understand that people believe this, but why do they do? Nothing in our society operates in a way that might imply this.