The Opportunity Cost of Drug Testing Welfare Recipients

Keith Humphreys points out the harm/waste of drug testing welfare recipients:

…welfare recipients, haven’t done anything wrong and have a low rate of drug use. … Further, while there is no evidence that drug testing welfare recipients protects the public in any significant way, regular testing of felony probationers coupled with modest sanctions reduces crime, including violent crime. …Given those realities, the continued interest of state legislators and governors in allocating scarce resources for drug testing to welfare recipients rather than felony probationers is as ill-advised as it is disappointing.

Why, then, is it such a popular idea?