You cannot guarantee me that a stoned doper is going to stay indoors, no more than I can guarantee you that a speeder won't get into an accident. How can you make such a ridiculous statement?
Just like a DUI or speeder may injure himself or others, so may a person on drugs, indoors or out. Drug users overdose and die. Drug users rob and steal to feed their habit. Drug users are a drain on the medical system.
Now, maybe that's a small percentage of drug users. But it's only a small percentage of licensed and insured (by law) DUI and speeders that harm others.
You can't guarantee me that a gun owner will keep his guns away from his children. Let's ban 'em.