Most conservatives realize that not every problem in the world requires a heavy-handed government solution. In fact, I think it's safe to say that conservatives know that the *vast majority* of problems don't need any kind of government solution at all. Hand in hand with that conservative notion is the idea that, as a general rule, people should be free to do what they want.
This is not to say that conservatives favor total anarchy or no restrictions on individuals- clearly they do not. It's just that conservatives understand the importance of keeping the government limited to a few narrowly-defined responsibilities, and using the government only when really necessary.
As a conservative, why do you believe that marijuana smoking by adults is such a serious problem that it calls for a complete government ban on the substance and punishment for anyone possessing it?
If you applied the same reasoning to any other substance or activity, would you be just as comfortable with the conclusion you reached?
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.