I work at a company where if you show up intoxicated to work, you're done. Alcohol is legal, yet I have yet to hear of any challenges to this policy.
I don't see very many companies worrying about if they can fire or not. If you are a driver, you're probably not going to be able to smoke. State legal or not. Pre-Employment drug screenings are probably not going to change. If you piss hot for THC, they probably won't hire you. Regardless if it's legal or not, they are free to make that choice. Random drug tests in the work place, aside from people who drive machinery, are pretty rare. Probably because it gets rather pricey to do so.
Don't forget, this is a "fire at will" state. You do not have a right to your job. You can get let go for just about any reason and there is little recourse in most cases.