A punishment may (or may not) be deterrent enough for the people who saw it. However, when you get a new class/userbase regularly or when new technology becomes available (including devices that can control your hardware remotely), an ounce of prevention is worth a pound of cure.

Perhaps using your watch as intended doesn't seem particularly mischievous in the moment. In this way, legitimate users/sys admins/script kiddies get into trouble because a sys admin didn't have reason to believe their users would be "particularly mischievous".