It should be one's own morality, yes. But I find it rather obvious that many individuals lack sufficient morality to act in the interest of others. Laws, and the punishments for breaking those laws, are supposedly meant to force the actions of the immoral to align with that of the moral as far as public safety is concerned. Of course, this is only true when the laws are just, but the prevention of people driving while high is a worthwhile goal, and it is reasonable to punish those that partake in methamphetamines while operating a large hunk of metal that can easily kill people.