Maybe he’s stupid, but I wouldn’t call him a dickhead.
What if the situation were just slightly different? Say I parked illegally, and I come jogging up to the cop writing me a ticket, and he says “if you eat a chunk of my beard, I won’t have your car towed.” That’s reasonable to complain about right? Even if I were actually guilty of the misdemeanor.
Coercion like that is wrong. The cop could have had him grind his weed into the dirt or something. I have friends where that ended up happening before it was legalized.