Crabs downvote because that's what crabs do. Their incentive is the good feeling they get when they stop someone else from getting something undeserved.

The Story of the Crab Bucket

A man was walking along the beach and saw another man fishing in the surf with a bait bucket beside him. As he drew closer, he saw that the bait bucket had no lid and had live crabs inside.

"Why don't you cover your bait bucket so the crabs won't escape?", he said.

"You don't understand.", the man replied, "If there is one crab in the bucket it would surely crawl out very quickly. However, > when there are many crabs in the bucket, if one tries to crawl up the side, the others grab hold of it and pull it back down > so that it will share the same fate as the rest of them."

So it is with people. If one tries to do something different, get better grades, improve herself, escape her environment, or dream big dreams, other people will try to drag her back down to share their fate.

Eliminating “abuse” is not possible and shouldn’t be the goal. Even those who are attempting to “abuse” the system are still doing work. Any compensation they get for their successful attempts at abuse or collusion is at least as valuable for the purpose of distributing the currency as the make-work system employed by traditional Bitcoin mining or the collusive mining done via mining pools. All that is necessary is to ensure that abuse isn’t so rampant that it undermines the incentive to do real work in support of the community and its currency.