Why not?
I see no reason why adults should be baby-sat by the government. Do we really need, or want, some huge institution telling us what is good for us and what isn't? Forcing us to do, and not do, things "for our own good?"
Can we not make these decisions for ourself? Are we not "big boys?" Are we not capable of paying the consequences for our own, perhaps bad, decisions?
So what if some people screw up by not taking the time to study what they are putting into their body. Stupid people will die and not breed - the definition of darwin evolution.