Why did they "have" to start taking drugs in the first place? If you take drugs and get addicted, that's your responsibility. Not anyone else's.
Well, some of them were under 18 at the time. As a society, we've decided you cannot really be held responsible for many of your actions when under 18. So it certainly is difficult to condemn teenagers to a lifetime of addiction because you were too cheap and on too high a moral horse to help them out.
But beyond that, in many cases, such as with student loans, we hold that society has not just a right to protect you from others, but to help enable you to improve yourself. Certainly, that seems cheaper to society than trying to punish people in prison for something they may wish they could give up.
Lastly, while you may wish that everyone was solely responsible for their actions, and their actions solely affected them, neither is ever the case. It's a good bumper sticker philosophy, but it falls apart once you start asking questions.