The reason he gets mad is because yes he does know how stupid it is and he is frustrated with trying to justify it. He is being defensive, very similar to a certain group of people who get mad when you challenge their politics. They know they are wrong, but are gripping tightly to their false narrative, because the shame of admitting they are wrong would be unbearable. This deputy made a conscious decision to go against his better judgement to harass people who are not doing anything wrong, he admits he is not enforcing any written law. So yeah, he knows how stupid the whole thing is.
This year ask him if there is a law that says you have to sign the paper. I would like to know.