This argument pops out in many societal issues, not only cheating on exams. Most people don't cheat and act in good faith but we make them suffer anyway becaus

This argument pops out in many societal issues, not only cheating on exams. Most people don't cheat and act in good faith but we make them suffer anyway because we will be damned if even one person cheats their way through the system, not on my watch! Make sure it's bad for everyone just because if someone exploits the system it will be bad for everyone. In the end there is a healthy balance and those who cheat the system will either fail later from the lack of skill/knowledge or, if they don't, it means it was sort of unnecessary anyway