It's the insistence that everything that people do must be compensated with money. People have spent years posting on Reddit for fun, without any thought to being paid for it, and now all of a sudden someone else is making some money so they're demanding that they should get their slice. And doing what they can to wreck their earlier efforts when they don't.
How does Reddit making some money licensing this stuff harm those of us who contributed to it? Is there any problem aside from "I wanna get paid!"?
If you did it over the API debacle then you're not one of the people I'm talking about here. This is about people deleting their content to prevent it from being used to train AIs.