I suspect if you disallow arrests based on these tests and require a lab followup, the tests will cease to be used entirely.

The police know the false positive rate and they'll stop wasting their time and rely on their training and instincts, instead.

There's an implication of automation bias here, too. "It came back blue, so I can just make an arrest knowing that the blue bag told me I should. Not my fault if it's wrong."

Pushing farther, if the law said that if there was a false positive, the arresting officer would have to spend one day in jail per day the suspect was jailed, no cop would ever dare use this test. That demonstrates the amount of trust they actually have in it.