Legalizing pot doesn't mean you can smoke pot anytime, anywhere you want to.
If your employer has a rule about drugs and you fail a piss test, you can still be fired. It's no different than alcohol. You can't show up to work drunk most places, or drink at work, either (Unless you work for an advertising company in the early 60s).
You still cannot drive while under the influence. Same as alcohol.
Tobacco is legal, but only assholes blow smoke at people or smoke in enclosed areas or right by doorways where nonsmokers can't escape it.
Rights come with responsibilities, folks. Most folks seem to think they have every right to do whatever they please without considering the responsibilities they have to others when exercising them.