What if by stopping that cardi, the officer saved 3 people's lives, that when they got addicted by that person's small doses, couldn't get more? What if that 2 units of cardi that the addict needs get stolen from him when he gets mugged on the streets? And what if that mugger just happens to drop a cardi pill in the governor's drink? And what if the governor then turned into a cardi addict? And then what if? What if? What if?
You can't play the what-if game as law-enforcement. It's a favorite tactic of protesters, and in-RP it can be quite the effective torch. But law enforcement would not realistically take that view. After all, if they didn't stop a person's "personal" cardi, it sets up a dangerous precedent. What is the proper amount of cardi for someone to be let through with? Lots of dangerous territory there, it is much simpler and easier on the officers to just ban it completely.
Edit: as to your point on the lawfuls' purpose... Lawfuls are supposed to protect society as a whole, not individual people. Oftentimes, protecting individual people lines up with that goal of protecting society. But when it comes into comflict, it would be stupid to protect a single person as opposed to a large group of people.
