Also, stupid laws breed contempt for the law.
When people discover they've been lied to about the deadly evils of marijuana, they may think that warnings against meth are also just 'crying wolf'.
Oh, believe me, there are dozens of reasons why the War on Drugs is unbelievably stupid that we haven't even touched on in this thread yet, and this is one of them. The War on Drugs is the most misguided, anti-freedom, damaging, and idiotic domestic policy since slavery.