I remember there was a stagette party on a Thursday night a few years ago for a woman at my office. The next morning, pretty much all the women in our department were clearly hung over, clearly impacting their performance for that one day (on the plus side, us guys who were the other 50% of the staff pretty much had the run of the place, ha ha).
Should all these women have been fired for the stuff they did on their personal time the night before? I don't think so. Now, if someone shows up hungover 5 days in a row, in a manner that impacts their work, then we have a HR issue. But that can apply to almost any personal-life matter, and if pot is decriminalized, I don't see why we have to be more puritanical about it than we would for any other vice (or virtue, for that matter).
Also, in my purely anecdotal experience, someone who smoked weed the previous night is usually clearer in the morning than someone who was drinking the previous night.