The legalisation of marijuana can be a huge windfall for the state. The state can regulate and tax marijuana to earn income - Colorado gained $5 million in tax revenue the first week that marijuana was legalised. Additionally, the state will save extra money by no longer having to spend money on the incarceration of those in prison because of marijuana related offences.
Theoretically, legalising any drug could be a significant source of tax revenue for the state. This is not a reason to do so. The amount in tax revenue legalising marijuana would produce does not alleviate the harms it would cause to individuals.
[P1] Legalising marijuana would both earn and save the state a lot of money. [P2] Marijuana should be legalised as it would represent a strong source of income for the state.
Rejecting the premises
[Rejecting P2] Something earning the state money is not a reason to allow it.