What someone does on their own time is not my business, as an employer.
UNTIL, what they do on their own time affects their performance or safety (or themselves of others) while on the clock. THEN, it becomes my business.
With doctors or nurses, the safety of their patients is at risk if they got high right before work. I'm not willing to let someone get injured or die because I did not want to invade the privacy of the doctor or nurse involved in their care.
I don't want a policeman, coming down off his pre-work high, mistaking the bubble pipe I have in my hand for a weapon and shooting me.
I don't want a fireman who was snorting coke before he began work, answering the alarm with an empty air tank because he forgot to check it in his post coke haze.
I don't want a teacher, responsible for my child's education, to be slurring their words or forgetting where they put their lesson plan because the shot up before class.
The safety issues with bus and truck drivers in this situation should be obvious and not require an explanation.