I seem to recall there being incentive to not down vote just for the hell of it to screw with people.
Downvotes effectively cost you money. Every vote (up or down) reduces your voting power which means when you downvote and then subsequently upvote something else, the positive effect of the upvote is weakened.
This mechanism is quite hidden so it may not have much effect on behavior.
It seems perhaps that downvotes have a disporportionate effect on ranking. I noticed that after the three downvotes on my blog post that had at the time 150+ votes and $900 of earnings (which ended up later cratering to $400), it was farther down in trending than posts with less votes and less earnings, as if the 3 downvotes were weighted more by that trending ranking. In other words, it appeared that the downvotes impacted the trending rank more than the earnings.