If your employee is a habitual pot smoker there is still the risk that one day he turns up high for work. Why would you take that risk if you can hire somebody who does not smoke at all?

Other than that, consumption of cannabis is correlated with a bunch of stuff that an employer may want to avoid (for instance increased impulsivity [1])

Given the fact that employers routinely choose to not hire somebody because of even the slightest misgivings, not wanting to employ a pot smoker can hardly be called irrational.

[1] http://www.ncbi.nlm.nih.gov/pubmed/25595054