Yes and no.
Drug testing addresses the last 3-30 (up to 90) days of an individuals life. It has NOTHING to do with them doing drugs ON THE JOB. Also will point out that things like coke and meth clear your system FAR faster than THC. Go figure.
Taking it a step further, someone who parties friday night can be 100% fine to drive your precious cargo. Can 'bad' behavior spill over? Yep. But a bus driver can also show up drunk to work.
Plenty of people use drugs and simply stop when they know they're changing jobs. It's basically a joke. Many drug testing facilities have lots of strict rules that they rarely or unequally enforce.
At the end of the day, you want someone sober while performing their job. Fine. 110% fine and understandable. I totally agree. Even random/regular drug tests (which are extremely rare) won't differentiate. Most of the time it's just a game of beating one test per job and then going on your merry way.
But we can do it FOR THE CHILDREN.