If the expected rate of known shoplifters is 1 in 10,000 then the human security team can keep a close eye on 10,000 people (one of whom is probably a shoplifter) or on just 2 people (one of whom is probably a shoplifter). The latter is much more efficient.

Without the intentional false positives they could simply throw out 1 individual with no need to keep an eye on anyone at all. That's enen MORE efficient, but it utterly ruins the lives of anyone falsely identified.