It's not about whether people want to see it or not

Yes it is, that's what Reddit is about.

it's about common courtesy

Common courtesy to whom? The original poster? He's getting credit.

But this thing here, this is just cocky. The posts are three hours apart

Yes, to share it in multiple places on the site at the same time. How cocky of him.

this guy here even has "Ent" in his name, which leads me to believe he saw it r/tress, it did well on upvotes, so he put on r/redditcom

Yeah, what's wrong with that? He knew it would appeal to non-Ents so he shared it on a more generalized subreddit, and because of this people like me got to see it. As I already said, the OP is getting credit for it. If you say something about stealing potential karma, I will reach through the internet and smack you. The OP could have crossposted it if he wanted to, and karma isn't actually worth anything.

Not even r/pics

r/reddit.com is a subreddit for anything. Either way, that point is a red herring. Whether or not r/pics would have been a better subreddit bears no relevance on whether or not this kind of crosspost is appropriate.