What makes you believe bomb and drug dog handlers want false positives? I just can't see what there is to gain. If I'm a bird hunter and my bird dog is making false points one of us is doing something wrong and neither of us is going to be happy until the problem is remedied.
Because a dog with a certificate not worth the paper it's printed on is an effective loophole to abrogate our 4th Amendment rights. The dog "Bono" on whose performane the recent USSC decision rested only found drugs 25% of the times he alerted and yet passed certification by his department with flying colors.
Right now there is a lawsuit by Nevada Highway Patrol officers that their superiors are using "trick ponies" in their canine units...dogs trained to alert on cue from the handler, not because fhey've scented contraband...resulting in unlawful searches and seizures for the agency's financial benefit.
Gee, I cannot imagine why.
Other than the big bucks to be gained by confiscation.
Because the fact that a car doesn't contain drugs doesn't mean it won't have something else of interest. For example, cops may know that people driving to or from a flea market are likely to have large amounts of cash. Drug-dog alert plus large amount of cash equals free money for the cop's department. Some victims may successfully sue to get their money back, but in most cases the robbers will get to keep it.