I don't think he's saying that everybody employed by the autoshop needs to be a straightedge teetotaler; he's saying "don't drink or do drugs at work. When you show up to work, you need to be sober and clean".
Actually, no, I work for a giant megacorporation and anyone with access to vehicle keys cannot have any drug record or any drinking related offense for insurance reasons, absolute zero tolerance (except for management employees of course). If a driving privs field circus tech gets an alcohol related offense even on personal time, even not involving a vehicle, they're out a job permanently, ditto anything drug related. At best they can apply for another non-driving position but that never seems to work out.
So I find it incredibly likely that a car mechanic driving cars in and out of a service bay and taking test drives after a brake job etc is almost certainly a zero tolerance environment.
This is more of a small business and as employees are hard enough to find in the first place; drinking off the job is fine. DUIs are not. Heck, knowing my Uncle, I doubt that he'd really care if you smoked pot or whatnot, you just can't get courts involved as then the insurance companies will drop you.
For work, it's the addictive drugs that are the real worry. Addicts steal things out of cars, they don't come in because they are on a bender, they show up high, etc. Drunks do this too, but they stay sober for longer. Most people relapse though and then you have to fire them. It happens a lot and for a lot of reasons, but they still have to go.
I'll note: my uncle is very much a rarity in auto-repair. If you go into the back room of most auto-shops, you'll find a fridge of Buds or Coors. Guys may have one or two during lunch. Its not that big of a deal typically. Also, Crank gets it's nickname because the Hell's Angels used to hid the meth in the crankshafts of their choppers. Hallucinogens are not common as they don't take the edge off like opioids or booze. Hucking dirty transmissions in 100+ heat while covered in grease and grime is not a good idea for your back. Its a problem, yes, but the US fleet is maintained by guys on god-knows-what and has been since Al Capone's rum runners. Check the writing on your transmission or oil-pan, in yellow grease pen it'll have the initials of the guy that last worked on it, his phone/shop name, and maybe a lewd drawing of him smoking something.
There's a big difference between "no drinking" and "no drinking relating offenses"; it's the same as the difference between "no sex" and "no sex crimes".
