It would be the same as for monogamous marriage versus polygamy, or homosexual marriage versus one man/one woman marriage, or sex with kids at 12 years old (or whatever young age) versus being an adult, or killing kids versus not killing kids, or prostitution versus no prostitution, or a whole host of other things that come under laws set up by society because of the moral stance it takes and the source for it.
I'm a little confused by your response. You haven't described why marijuana smoking is a particularly bad problem nor given any reasons why it should be lumped in with the other things you mentioned.
Do you believe in a government response to marijuana smoking simply because you believe it is immoral? If so, what makes it immoral?
If immoral, what separates marijuana smoking from the vast majority of other immoral activities that the government should NOT regulate?
Or do you believe the government should be restrained only by the sentiments of the community (expressed through majority vote) when deciding which immoral activities it should restrict?