Jose, The rule states that when two singular subjects are connected by 'or' or 'nor' a singular verb is required and in current use that's the way it is usually done. You are familiar enough with English to know that writers do no always follow the rules. Apparently the professor feels that the idea of plurality is suggested by the context of the sentence. He is probably thinking...this, that, or both. In other words...Reward, punishment, or both are meted out quite independent of human interference.

Agreed with Alan, you still have two options. If the professor wrote "reward and punishment", that would mean they are given out at the same time, not as two possible options. Adding "either" does separate them and you would then use "is", but "reward or punishment" does not equal "either reward or punishment".

This is because both are given out. However, the subject receiving the reward or punishment only gets one or the other, so they used "or" instead of "and".