Actually I am against smoking pot, just not against people who do it. I guess my only point is THEY MAKE FUCKING PIZZA. Do I want my airline pilot high? Absolutely not. Do I care if they guy throwing cheese and placing pepporonis on my pizza can't even spell his own name? No.
Also, I believe the thing of drug testing everybody to find one person is bullshit. You might get the guy fired who had a puff at a party over the weekend fired for no reason. At a mechanics shop or a dealership you don't all get drug tested if something goes wrong, only if you wreck a car or some shit. Then there is an automatic drug test for THAT person.
Plus, I believe everyone will think he is a tight-ass for a boss. I think the easiest and highly-effective way to resolve it would be have a meeting and just spell it out in a friendly manner, "Look, I know someone was smoking pot in my bathroom the other day. If you're going to do that shit, take it out of here because it can get MY ass in trouble and then I would be forced to fire one or all of you." The efficiency of someone knowing that someone else is aware of what they are doing will either lead them to be A. Super careful about smoking pot at work or B. Stop doing it altogether. That would mean your problem is solved because either B happens or you won't know about A.