"According to state data gathered by ThinkProgress, the seven states with existing programs — Arizona, Kansas, Mississippi, Missouri, Oklahoma, Tennessee, and Utah — are spending hundreds of thousands of dollars to ferret out very few drug users. The statistics show that applicants actually test positive at a lower rate than the drug use of the general population. The national drug use rate is 9.4 percent. In these states, however, the rate of positive drug tests to total welfare applicants ranges from 0.002 percent to 8.3 percent, but all except one have a rate below 1 percent. Meanwhile, they’ve collectively spent nearly $1 million on the effort, and millions more may have to be spent in coming years."
Because it's a scam to scare you into believing the government has a right to dictate what you do with your private life.
source
http://thinkprogress.org/economy/2015/02/26/3624447/tanf-drug-testing-states/