This seems rather at odds with your first statement. Unless you're referring to limited medical use, I don't see how you can square "This will mess you up for life" with "It should be legal anyway".
The idea is that it's almost as easy to attain as if it was legal, however because it's illegal there's nothing to regulate the industry to protect the users. Buying weed is as easy as buying beer, but when you buy beer you don't have to worry about whether it's laced with something that's going to have some rediculous effect on you.
I'd imagine that some dealers would think the idea of lacing stuff with something to get people addicted to their specific product is a great idea. With weed however that sort of thing is pretty much non-existant. Anyone who can do it properly would move up to bigger fish, anyone who can't do it properly makes their customers sick and goes out of business.
The "high" one might get from snowboarding doesn't cause them to lose proper judgement, motor skills, etc, etc... and thus, it is infinitely safer than any artificial high caused by a substance (marijuana, alcohol, or worse)
But besides that point, regardless of marijuana's potentially harmful (or lethal) effects, it also causes a HUGE problem... The one he's facing right now...
It can keep you from getting work... Businesses don't hire people who use drugs because drug use is a liability... It can lead to poor workmanship, accidents on the job, and so on...
Using drugs therefore hurts you in a way that's even worse than physical...
And continuing to do so, and trying to find ways around the detriment is irresponsible, stupid, and selfish...
But hey, if that's your thing, then go ahead!
[Edit:] And you might say "but if it only keeps you from getting a job, then it's still only hurting you!" Wrong... Unemployment hurts EVERYONE... What happens when you can't find work for a long period of time? You end up living off of someone else's money...
And what happens if you manage to sneak past the tests, and get a job, only to get hurt on the job if you "accidentally" go to work high? You've now cost that company thousands of dollars, and helped to drive up insurance costs for EVERYONE else...
Sure, smoking pot doesn't guarantee any of this will happen... But it certainly raises the probability...