A couple of reasons. 1. I suspect that Drug dealers and traffickers are getting huge margins. I expect that the pharmaceuticals could produce a quality product for a lower price ... this would leave room to tax the sale & production without raising the price to consumers. The tax revenue could be used for drug education and rehab programs. Much like the DNR uses revenue from the sale of hunting & fishing licenses to fund wildlife preservation and fish planting. 2. I would consider the tax on drugs a "sin" tax .... same as the taxes on cigarettes, alcohol, lotto tickets, & casinos. I don't believe that firearms and ammo fall into the same category. Of course there are liberals that think that guns should be taxed more than other products.
I also have some thoughts on business and personal taxes but this is a thread about drugs. 
Another "tax and spend" solution. Isn't that how we've ended up with the onerous government we have? This seems to be the standard reply in any discussion of the de-criminalization" of any activity. "let's legalize it and charge a tax (or a fee as the repugnicans call it).
An what makes things a "sin" to be taxed?
"And the Lord found that the plant of the tobacco to be not one to be subject to fire. Therefore thou shalt be cast upon the rocks for all eternity if thou shall puff." Spartacus 4:17
"Whosoever shall make the wine not of water, but of berries and fruit, shall not inherit the Kingdom of Heaven." Spartacus 4:22
"The malt is vile. To consume it in mass quantities, along with tubes of beeflips (and other meat byproducts), in the coliseum while idolizing gladiators is unclean." Spartacus 9:6
