When people are made to believe that certain actions will be rewarded and perform them, they must be rewarded.
If not, if some other person gets the benefit or their contribution is ignored, they stop.
If workers are to get free lunch for arriving early and they arrive early and don’t get that free lunch, they are angry. They feel used. They not only stop arriving early, but know that any kind of ‘reward’ promised won’t be fulfilled.
Their interest, trust, is gone.
When students are to believe that scoring high on a test ‘wins’ something and they get nothing, why bother? Do as little as possible to get by.