Why would there be a need for a black market if people are getting it cheaper. Say there buying cocaine from Columbia, they get it very cheap and they add there tax on top. That would make it lets say £15-£20 a gram. Why would people pay £40 off dodgy dave down the pub. Making the drugs and selling them cheaper would get rid of the black market.
And why shouldnt they make money off it, they make money off everything else. Tax in this country is a joke and always will be.