Under the Australian CASA "DAMP" rules, everyone is testable except pax. It is common here for the testers to turn up at an airport and test everyone on site including engineers,cargo etc.
If you have an aviation business, then you can designate non aviation areas, so office staff for example are not covered.
So in theory a fair system. From the results that they are getting and anecdotal evidence, there is a small drug problem in aviation, particularly with the younger members. Older ones are still getting caught for alchohol but not so many.
Wunwing