smoking pot (or doing alcohol, or having neck tattoos, or being against gay marriage, or anything else), is a personal choice that nobody should be FORCED not to do.
Employing people , is a choice made by a private entity. If you CHOOSE to smoke pot, an employer should be able to CHOOSE not to hire you.
Again, the market will correct. If you don't want pot smokers, and all the good, skilled workers looking for that position smoke pot, then you wont' be able to hire a qualified candidate.
I have nothing against legal pot, and indeed anything that one chooses to do with one's own body.
But I abhor the thought of the government being necessary to step in and force another private entity to endorse and accept your life choices.
Generally, if your response to a situation like this (be it pot, gay marriage, etc). is "we should have a law" or "we should change the law so that...", then the truth of the matter is the government is already too involved. An employer should be able to determine themselves if pot in someone's system (which, as stated earlier, doesn't necessarily mean that person is impaired given the length of time it stays in your system) is a deal-breaker for employment.