It used to be my job to pull data for a large state's prison population. I can tell you unequivocally that it is NOT rare for drug users to be imprisoned. In fact I can say with certainty that most of the people attending prison use drugs.

Most low level drug dealers are generally paying for their own supply.

At my (very highly regarded) university in the UK you'd see this all the time. Pick up bigger bits, resell to friends, keep the "profit" for your own fun.

Drug dealer meaning they reached some arbitrary definition that has only a passing relationship with dealing drugs.

The term drug dealer is a bit vague, keep your weed in two bags? Must be a dealer.