They tried this in Florida and found that the costs of the program—the tests, the labs, the technicians, the administrators—outweighed the savings in excluding people who failed by a large margin.
So what, if drug testing costs more then welfare benefits! I for one do not want to enable drug use! Why should I, against my will and against my moral principles, give money to people using drugs. It is depraved and cruel act to give a hand out to a drug user, I am willing to pay more in taxes to stop the drug users from getting money to pay for drugs at my expense. On the other hand, I am perfectly willing to help those who need it if they are willing to help themselves by being drug-free and ready to work. (Further, people if they have money for drugs, then they have money for food.)