Because they own the content and control over how it is distributed. They chose to grant it to reddit, and you have decided that also grants you a license, although it actually doesn’t. You are taking it anyway which violates their rights to control their content and who may distribute it.

I agree with you that the users are being exploited by Reddit, but that doesn’t mean that we can come in and further exploit them in the interest of trying to avenge them. That should be (and legally is) their choice to do or not to do.

And I also agree with you that the exploitation is the issue. The API stuff just shined a light on it (or should have anyway, but I also agree that it became just a footnote in the story).