That isn't true.
http://smoking.ygoy.com/effects-of-s...-on-the-brain/
http://www.psychologytoday.com/conditions/nicotine
However, I do see the point you're going after; namely, that cigarettes do not affect the mind in the same exact manner as marijuana. I do not condone things like driving under the influence, naturally. At the same time, I understand that there are plenty of legal medications that can produce side-effects unwanted in a work environment, and yet, they are tolerated—they're legal, after all. In direct comparison, for those who would smoke marijuana for medical reasons, being unable to use their medication at the workplace would be unfair treatment. Would they have to drive home, smoke, and then drive back? Where is the safety in that?
This kind of discrimination would also negatively impact efforts to remove marijuana detection from mandatory, job-related drug testing. If a job may bar marijuana use, they may also bar marijuana users. Due to the current federal law, this is the way things work. Additional obstructions related to marijuana-friendly policies in the workplace would only compound that.