It isn't the full story, but part of the acceptance of "only poor people get busted for drugs" is based on the idea that the poors don't deserve any "fun" at all. Obviously given addiction issues drug use is a bit more complicated than "fun," but people who have no problem shelling out $50 for a bottle of wine are horrified to see the poors with a six pack of cheap beer. Likewise, upper middle class people who for one reason or another rely on drugs, legal or not, think the poor, with their slightly more acute struggles, must have purity of essence.
