People make bad decisions all of the time.
The liberty to make bad decisions is a part of freedom.
I am not going to argue that marijuana is good or alcohol is good or the use of any other drug is good.
Freedom means allowing the individual should be able to make a judgement and choose for themselves. Otherwise, what is to stop nanny government from regulating caffeine (another drug), aspirins, etc off the market because there is some potential harm or risk of abuse?
The only thing that goes along with these freedoms is consequences. If people make bad choices, government should not be there to provide any support or rehab.