The problem is many of these sheriff's departments are defining "probably cause" as "has a Colorado license plate". Considering how easy it is to transport marijuana and not get caught, I doubt the increased arrests are all solely due to the greater supply. That's basically why the whole bit about the sheriff's departments not making the laws doesn't really hold up. For comparison Pennsylvania gun laws are far more lax than in New Jersey or New York, but that doesn't mean police in those states constantly pull over and search Pennsylvania cars and assume all are carrying guns.
I take your point, but that dog just isn't going to hunt in court.