Person A smoking MJ does not harm anyone else. If person B is harmed by person A while smoking MJ, punish person A for the action, not the smoking. How about that?
If that's the rationale, then how do we justify making it illegal to: go 95 in a school zone, shoot at someone and miss, drink and drive, let my pit bulls roam free, build a large bomb, or put a mine field in my front yard. What's wrong with attempted robbery (or attempted anything)? Why were people so bent out of shape when Michael Jackson hung the baby over the balcony? Hell, he should be allowed to juggle three of them.
With that thinking, do I qualify for honorary Libertarian?