They call it "hustling" and "growth hacking", and I suppose there might be good ROI on this when you're just starting up and trying to do some "organic advertising" in a niche community/subreddit. Those couple extra upvotes may be the difference between drowning unnoticed vs. staying on the front page long enough to gather initial interest, and spark a discussion that keeps it up for a day or more.

I still believe it's unethical behavior, though.