Addendum

At least in theory, drug testing disincentives people on welfare to consume drugs, which should in theory be good. And yes, puts a break on some people who probably do not deserve welfare as much as others do. (Hard to prove, to be sincere - but an easy theory to follow.)

I personally would take into account that many people start or increase drug consumption in reaction to negative life events. And I would also take into account that alcohol is generally legal, and cheaper, and yet more personally debilitating and socially dangerous than illegal drugs are. So, I see the rationale of "society shouldn't pay your habit" and I know it applies to some people - what I find hard to proof is whether the proportion of each population; or the comparable effect of alcohol.

I will also go and say that there probably is a racist inspiration for this idea, same as every drug law idea ever has had a racist origin...