Sounds fishy to me - can you cite a snigle specific instance of such a finding of fault? And why doesn't the same risk apply to employees using the mind-altering drug alcohol - which as far as I know is rarely if ever screened for?
You obviously have a fight to pick and are looking for someone to fight with. Not me - I dont have a dog in this fight. Here are some pages for you go after someone:
http://www.acde.org/employer/DAwork.htm
http://www.osha.gov/SLTC/substanceabuse/index.html
http://www.osha.gov/pls/oshaweb/owadisp.show_document?p_table=OSHACT&p_id=3371