Why Can't We Drug Test Welfare Recipients? I Got Tested For My Job

Friends of mine who are somewhat liberal ask me, Why can't we drug test welfare recipients? I got tested for my job. It's true, many people are tested for drug use before being employed. Even the government tests their employees. That still doesn't make testing welfare recipients constitutional. If the Fourth Amendment means anything it means the government has to have probable cause to test you for drugs. Having a bias toward poor people, thinking they must all be dirty and on drugs isn't probable cause and it also doesn't match with the actual evidence of drug use by people on public assistance.