well i do believe its got alot to do with insurance,ect.
get hurt on the job,test dirty, bye bye any benifits ya had earned.
they will say YOU violated the rules and was stoned,TOTALLY your fualt even if a crane fell on you they'd claim,you would have seen it and ran if not high

this shit needs fixed,
alot of places jumped on that bandwagon,its so sad
good luck