I assume it's more about predicting these things before employment begins - someone might look ok at the interview but if you found out traces of drugs in his blood that could be an indicator that this is not their usual state.
Basically we're talking about low-skilled jobs that always have plenty of applicants, filtering a bit too many people isn't that big of a deal when there're still plenty of others remaining afterwards.
It's just a cheap initial filter, as long as it's better than completely arbitrary it's probably beneficial to have.
Not that it matters much, and this is only anecdotal but I had friends who would smoke pretty frequently. A lot of call centers, and retail spots would drug test. My friends would still get the job if they got past the initial interview. It seems like most companies are more interested in harder drugs, which I can understand.
On topic: I think that if you're looking for a cost effective way to live, do what someone said earlier and move to a less prominent neighborhood. This seems like it would be more along the lines of "fun to do for a couple weeks".
Basically we're talking about low-skilled jobs that always have plenty of applicants, filtering a bit too many people isn't that big of a deal when there're still plenty of others remaining afterwards.
It's just a cheap initial filter, as long as it's better than completely arbitrary it's probably beneficial to have.