"You won't work anywhere in the financial industry"
... yes, well. I'm glad of that, personally.
Or you can say companies started doing it years ago to get us accustomed to the drug testing so that one day they can start doing DNA screening and we'll just shrug our shoulders.
I've held a number of jobs (in the US) over the years and I only had to do a drug screening twice.
Sure, if someone comes in to work too inebriated to do their job, and you let them carry on anyway, that'd be your fault. But if you can't tell they (say) had a joint last night, then what's the problem? For one thing, if you can't tell, it's unlikely to be materially affecting their work; but regardless of that, you surely can't be held liable for something you can't detect? (As you said yourself, periodic drug tests are not a reliable detection method.)
I don't get US liability law. I really don't.
... but don't worry, I'm not in that "they're getting us used to testing!" tinfoil-hat-brigade. It's just, if your private habits don't affect your work, then they're none of your employer's business; if they do affect your work, then your employer shouldn't need a drug test to notice. Trying to regulate your employees' behaviour when it doesn't affect your business is busy-bodied puritanism, nothing more. IMO.