Loads of folks make their living off preying upon folks busted for drugs.
Lawyers, drug counselors, probation officers, erc.etc. That is the reason drugs like marijuana are still illegal.
Also do you want crazies driving on crack? Or heroin?
Drug laws have prevented almost none of those prone to use drugs from using them, so if they were going to drive while on crack or heroine, then chances are they do anyways.