I don't know about the not getting prosecuted bit, but drug taxes are real -- see eg http://www.time.com/time/nation/article/0,8599,1578857,00.ht...
http://marijuanastamps.com/state-stamps.htm
Mostly, it's another way for pigs to steal from people. The idea being they can not only throw you in prison for dealing, but then use tax forfeiture laws to take everything you own.