Interesting discussion.
"Legality is irrelevant"
This is good, because I'm sure we can all agree that just because the "law" constitutes something does not mean it is in any way best or even rational. Laws begin to function ideologically when people begin to think that there is somehow an inherent, universal truth behind the law. However, this isn't happening here since legality isn't an issue.
Certainly; I've spoken on a number of occasions on this forum about the absurdity of drug laws and the nature of the legitimacy of law, so I was surprised this was even brought up.
"This is not a moral judgment"
Also good, because it's asinine to try and argue that there is an inherent moral flaw in drug use. I'm sure everyone agrees on this as well.
Indeed, I never intended to speak from a perspective of morality, or at least not pop-culture morality.
"Irresponsible"
This is where the crux of the argument is, and it's the most interesting part; irresponsible for whom? Certainly for the user; that goes without saying. Dodens mentions that smoking in one's room isn't "smoking responsibly;" but I contend that it is certainly more responsible than those who smoke on their drive to work. Thus, people can be responsible toward others while being irresponsible toward their own bodies.
Naturally, it's more responsible, or more accurately less irresponsible to take precautions in order to prevent their own actions from affecting others. I will continue to argue however that recreational drug use by its very nature is irresponsible at the very least to the self, although it sounds like you do not disagree with that.
Smoking might be an unhealthy act, but so is eating poorly and not exercising. Plenty of pot smokers eat very well (there are tons of vegan pot smokers), and many also exercise. Instead of labeling smoking as an "unhealthy" habit (since plenty of people who don't smoke are far less healthy than people who do), let's label it a "risky" habit. Now, risky habits also include skydiving and mountain climbing, acts that pose no danger to others, but do to the subjects themselves; these acts are also healthy (especially mountain climbing, which is a form of exercise). That doesn't mean that risk is not involved; so, are people who engage in these types of activities also irresponsible? Health insurance companies think so.
While I think that it's fair to bring up other aspects of lifestyle, I would say that it is difficult to find a truly symmetric comparison; for example, eating unhealthy foods still provides the body with nourishment and sustenance, while not exercising is not actually an "act", but rather the lack of an act, so at best it is in this sense passively irresponsible. I would also make a distinction between acts that run the risk of physical bodily injury like skydiving and internal organ damage like smoking as separate categories of irresponsible behavior, although both are worth discussing in some forum, if not this one. Certainly there are a plethora of acts or non-acts that we perform every day that are irresponsible to some degree.
Now, the central component of the argument is whether or not it's okay for parents to manage their teenagers' money. I have two questions: 1) do they know you've taken drugs (i.e. bought drugs with your money)? 2) how did you earn this money?
If they don't know you've done drugs and are just managing your money because they fear you might act responsibly, that's reprehensible. Institutions reserve no right to intervene simply because they assume someone might act irresponsibly. Now, if they caught you smoking or know you do, managing your money might be an act of trying to protect their own assets (i.e. the money/time they've invested in you thus far); ergo, I can't really blame them for doing so.
Not to speak for him, but I do seem to recall that his parents are aware that he has done drugs, and I also recall that he has had some psychological issues in the past, as well as an at times strained relationship with his parents, the fault of which rested with both parties. If his parents suspected that he would use the money to buy drugs, then I don't really have a problem with it as long as he remains under their guardianship and are responsible for him.