Should employers be testing for something if it's legal?

Back in May, I posted about how more employers in New York were dropping drug testing.  The topic has come up again in the news, and the area still seems to be vague as New York seems to be getting closer to the legalization of marijuana.   In a time where marijuana is being legalized medically, and in some of our neighboring states for recreational purposes, it's questionable if marijuana should still be included in drug screening for employment.  News 10 abc recently posted again about employment testing for marijuana, in the setting of legalization, and how it's really still a grey area.  There still seems to be back and forth on what policies should be, and what New York should do, and it's interesting to read some of the quotes from News 10 abc.  Frankly, I think that no one should be under the influence while at work ever- so there may be times where a drug test is needed if there's a question if someone's impaired. However when you're looking at excluding people from a job solely for smoking pot?  A joint in the evening does a lot less damage than someone being hungover in the morning for work.  What are your thoughts?