We actually can. There is a rule in TOS:

"17.1. When accessing or using the Services, you agree that you will not commit any unlawful act, and that you are solely responsible for your conduct while using our Services. Without limiting the generality of the foregoing, you agree that you will not:"
...
"17.1.2. Use our Services to pay for, support or otherwise engage in any illegal activities, including, but not limited to illegal gambling, fraud, money- laundering, or terrorist activities."

Encouraging people to upvote for some reward which will be granted to the random picked user - isn't it gambling? :)