I agree it is stupid to smoke weed, especially in this scenario. You keep focusing on the scenario "changing" and not the point. They should not be going out and drinking or smoking weed, whether or not one is legal and the other is illegal.
If you can not go to meetings about drug and alcohol for your career without doing either, you do not have good judgment and your employer will see that. Is it a big deal if you have talent, no. But if you are a rookie and on the cusp it would not be wise.
Let me outline a scenario I brought up earlier. If your boss takes you lunch, and you order 2 shots of Patron will that look reflect well on you? You will not be doing anything illegal, you will not be drunk, but you will create a perception. That goes along with the point I was making about going out to the clubs. So what if I added on to the scenario to make a point? One is legal and one is not right?