"children" under 18 don't have the same judgment as adults (18 is an arbitrary age, but has some precedence).
It is justified to protect "children" from themselves, by making it illegal for them to smoke.
Adults, on the other hand, are allowed more freedom. In effect they are assumed to know better than to engage in self destructive actions (because they can judge what those things are better than anyone else).
As a society we have chosen not to protect adults from themselves to the same extent.
This is a varient of the "don't have the same judgment" argument...so I'll address your counterpoint.
The "child" is assumed to not know better than to smoke.
In the case of the adult, the person is assumed to know the risks involved and be able to choose what is best for themself.
As far as using "Mechanical Turk", that's a very interesting use for the service.