it is VERY expensive to test to see if its real. a normal test consists of drug testing an testing for a vitimin or 2.
it is very rare that a lab will see synthetic unless they pay the extra to look for it , an they never do.
i would imagine that some of these companies realize they will lose half their squad to pot , so they choose a cheap lab to test just for that reason. They figure if you got enough sense in your head to "cleanup" for a test then your proly not doin anything that bad to begin with.