I feel like calling it stealing is too aggressive. Reddit is a link aggregator, so a lot of the initial content is a link to somewhere else. It's technically not Reddit's OC to begin with, no?
Then, as far as reposting things like images, similar argument—there's a lot of reposts of that content on Reddit anyway. It's a grayer area than the former, imho. But as long as you don't rebadge it like 9gag does, I personally don't see anything wrong with it as a way to bootstrap activity.
Hell, that's what these massive AI companies did with their training days. Gotta play a little dirty I guess.