There does seem to be a lot of "this is 'scientific' so it's good" in law enforcement. A lot of forensic techniques are just BS, but are accepted because they're "scientific." Or look at the recent article posted to HN about how polygraphs are nonsense.
Of course, it's the job of the police and the courts to come up with an answer, so we should be able to let them do it. But it seems they can't be trusted to reason about it well.
If all you need is your own suspicion, it could be easy to let one's bias against purple people run rampant, and search every purple person's car and nobody else's.
If you have to call in a dog which you believe to be effective (even if it may not be) then you may only go to that trouble when you actually have some legitimate suspicion, rather than just going based off your anti-purple racism.
One wonders if we have the procedures we do because it gives a thin veneer of scientific respectability over a system that might otherwise appear biased, racist & otherwise distasteful even if it's statistically valuable.