I just read Tuesday's NYT article on the subject. The assumption that poverty is probable cause to suspect criminality is, of course, a disgraceful violation of Americans rights. I could go through the trouble of digging up one of the many studies showing that the poor are neither any more or any less likely to be criminals or addicts than the rich, but there's no reason to, since to repeat the obvious the right-wing authoritarians pushing these laws don't give a damn about actual reality but only the right to define reality.
In some ways this reminds me of laws enshrining hunting and fishing in the state constitution, or proclaiming the right to carry a concealed weapon into a Kinko's while weearing a full-body Zuba suit. You see a lot of this symbolic governance when conservatives are in charge or state legislatures. (Practical governance is of course beneath the dignity of their knightly selves.) In the case of various hunting and gun-protection laws the purpose is to give institutional approval to the notion that herosim is the norm for Real American Men; that True Men are eternally primed for violence towards the end of defeating evil or providing food for their clans. It provides reassurance to hunters, fishers, and concealed carriers; people who in the back of their minds know themselves to be a small portion of the population yet for the most part strongly deisire to be the universal standard, that their proclivities are the True American Way, that it's the city boys who vastly outnumber them who are the deviants. People who rail against big government the most are often the most desirous for state authority to pat them on the heads and tell them what good boys they are.
The purpose of drug testing for welfae applicants is similar. It is to will the notion that poverty is proof of depravity into reality by having it officially enshrined by institutional authority. Never mind that some faggy sociologists say it isn't true, the state of ------- says it's the truth, therefore it indisputibly is. 'We', are America's responsible parents, singlehandily keeping it afloat against the riff-raff. 'We' are better then them and entitled to subject them to whatever discipline we see fit. 'We' are America. those filthy Jiggaboos just live here. God bless America.