In Oregon and some other states legal recreational off duty pot use can be a fireable offense if there is a zero tolerance drug use policy ie a DFWP/drug free work place.
http://www.oregonlive.com/marijuana/index.ssf/2015/07/oregon_employers_free_to_fire.html
http://www.oregonlive.com/marijuana/index.ssf/2015/07/oregon_employers_free_to_fire.html