I believe that overall the decision is a good one, there are no real good reasons why cannabis should be prohibited. At the same time, I strongly hope that the cannabis smokers will be considerate of the people around them. It is quite a foul smell if you are not partaking yourself.
Is there any good reason why alcohol should be prohibited?
Regulating the quality of the cannabis on the market is a large win for public health among users of cannabis. In a black market, you can't be really sure what you're buying and people are incentivized to buy a stronger product even if they don't actually need the stronger variant.