I think you might have an exaggerated idea of how addictive illegal drugs tend to be compared to alcohol and tobacco. Heroin and Meth actually are more addictive[1] than legal drugs, but not magically so. Still, banning just those two drugs would be a coherent policy, unlike our current laws that also ban Cocaine, Marijuana, normal Amphetamines, LSD, etc.

[1]http://www.americanscientist.org/libraries/documents/2006451...