Legalising marijuana benefits the black market
Legalising marijuana serves to make it more popular, which only serves to help the organised crime units that profit from it.
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Legalising marijuana does not decrease black market activity. Instead, organised crime in Colorado has skyrocketed since the legalisation of marijuana, leading the Drug Enforcement Administration to state that “since 2014, there has been a noticeable increase in organized networks of sophisticated residential [marijuana] grows in Colorado that are orchestrated and operated by drug trafficking organizations.”
The black market, of course, will not go away overnight. However, once marijuana is legalised over time we can expect the black market to shrink dramatically as they are no longer able to command the same prices they did when marijuana was illegal and they were therefore able to charge a premium for the risk involved in its production. The reason that crime increased in Colorado was not the legalisation of marijuana there, but the fact that it was still illegal elsewhere.
[P1] Legalisation of marijuana has been found to increased rates of organised crime. [P2] It should not be legalised in order to avoid this.
Rejecting the premises
[Rejecting P2] Increase in crime rates is due to the illegality of marijuana in other places, so is not a reason to keep it illegal.