Maybe it seems unlikely, but that's how it usually happens. Do you think people are introduced to Vicodin by drug-dealers, by and large? What usually happens is this: someone is injured, in a car accident or whatever, and is prescribed painkillers to ease his/her recovery. Well after the recovery period is over, he/she continues to need Vicodin (or whatever it is) because painkillers are highly addictive.
You're probably right that painkillers aren't as bad if used in the amount originally prescribed, but the problem occurs when that doesn't happen, which is often.
No doubt. But like I said before, this becomes an issue regarding the user, and not the the doctor. I had a status about getting my wisdom tooth out on Facebook. First post, merely minutes after it was initially posted was from a kid I hadn't talked to since high school in 2007. He said "let me get your prescription." Didn't ask if I was on any, he simply assumed and was on the prowl for them. Fortunately, I didn't have to break his little heart and tell him I didn't have any because he fucked off after a few of my friends sat there and humored him.
He wasn't a druggie in high school. But since college started, he became more and more into the legalization of marijuana and absolutely idolized drug references in music, namely Eminem's. I don't know him that well, but I don't remember him being injured recently or badly enough to warrant Vicodin. I can only assume, which is a bad thing, is that he's copying what he hears in music and feels satisfied having done what he hears about. The point is, pill poppers don't even go to doctors for their pills anymore.
So why are doctors being portrayed as dope boys and not simply as doctors? They certainly don't tell patients to take abnormal doses of their medication. Good, ethical ones certainly don't. So they're not for the abuse of drugs and they don't vouch for the legalization of marijuana based on the claimed "health benefits" of it. So who's left to validate research done by parties with interests in the legalization of it? Have there been independent studies into it? Surely those wouldn't go unheeded by the general public, let alone the government, especially if there's money to be made to regulate it.