Because arguably, the reason you shouldn't put these substances in your body is because doing so can be unhealthy. Criminal charges and prison are definitely unhealthy.
Making a person's diet a matter of law just goes against the values of many people. I'm well aware of the fact that some compounds lend themselves to addiction/habituation but so do many legal ones. You don't see people throwing winos and pack-a-day smokers in prison unless they otherwise break the law in a way that infringes the rights of someone else.
Likewise, if someone fails to use intoxicants or other recreational drugs responsibly and develops a debilitating habit, it makes more sense to do as we do for people who want to quit drinking or smoking, not as we do for people who rape and murder.
Making a person's diet a matter of law just goes against the values of many people. I'm well aware of the fact that some compounds lend themselves to addiction/habituation but so do many legal ones. You don't see people throwing winos and pack-a-day smokers in prison unless they otherwise break the law in a way that infringes the rights of someone else.
Likewise, if someone fails to use intoxicants or other recreational drugs responsibly and develops a debilitating habit, it makes more sense to do as we do for people who want to quit drinking or smoking, not as we do for people who rape and murder.