I have question concerning possible laws in the workplace.?

My friend has a part time job at a certain business. I recently quit because of issues with the general manager. He showed me a picture of the most recent employee contest to be held in July to be the top seller of a certain kind of item. The prizes included some gift cards, tickets to events, and two weeks of not doing closing duties. At the bottom it said, "The bottom 2 sellers will not get scheduled for 2 weeks." Is this even legal to do? The contest is fine, but not scheduling an employee for not selling as much as others? There are definitely factors in that. No one is going to get the same amount of hours as everyone else, thus destroying the chance to equally win the contest and save their paychecks by not being removed from the schedule for two weeks. Can anyone shed some light on this? Ask questions and I will be sure to answer.