To clarify: The misunderstanding lies in the reconciliation of pre-ban tax receipts and post-legalization tax-receipts and how they would be materially different if taxing is still taxing.
1) Black markets create higher prices. (increased risk = increased return)
2) Prices go up potential tax receipts go up.
3) Banning free market activity allows for the elimination of current competitive producers. (reduced supply increases price if demand holds steady)
4) Banning free market activity allows for later legalization of winners through licensing. (kickbacks)
5) Regulation through legalization creates government jobs which increases political power.
Tax-it-to-have-it schemes benefit politicians and governments not people.