Because on those devices the fingerprint is only used as a second factor (the first factor being physical possession of a particular device).
If Apple used fingerprints like law enforcement do, that is running a fingerprint against the big DB of all prints, then they would certainly get some arbitrary matches.
late edit: how often do people try to unlock your unattended apple device?
Another way fingerprints are used is to compare fingerprints found at a crime scene to people who have been identified using other methods that have nothing to do with fingerprints.
E.g., if you have someone murdered in his office when working late at night, and security cameras show that five other people were in the building at the time of the murder, and you are able to get a good set of fingerprints off the murder weapon you don't need a database. You take the fingerprints from all five of those other people, and if one is a very good match and four do not match, you concentrate most of your effort on the one that matched.
It's actually quite similar to how DNA evidence has been used and misused. I don't know how they compare DNA nowadays, but when DNA evidence was first making waves getting people convicted they only compared two DNA samples at a few points. For a given sample there would likely be several people in the country that matched.
That's fine if used right, like fingerprints in the earlier murder hypothetical. Narrow it down to only 5 people who could have committed the crime, get a DNA sample from the crime scene that must be from the criminal, and if that matches exactly one of those 5 suspects it is strong evidence they are the criminal.
Have no suspects yet, run that same sample through a database, get exactly one match, and conclude that must be the criminal. Totally bogus. A lot of people were convicted in the early days of DNA matching that way.
The database method can be made sound, but only if the database includes everybody. Match against a database that includes everyone, only get one hit, and you've probably got your criminal. But if the database includes everyone you are likely to get several matches.
As I said, that was how it used to be. I know DNA sequencing has gotten faster and cheaper over the years, but I have no idea if routine forensic DNA matching now matches enough to make matches unique except in the case of identical siblings.
Do you think the forensic prints would meet the standard required by Touch ID?
Given how much R&D goes into making better fingerprint scanners, it seems odd to then claim that a fingerprint taken off of a victim has anything near that level of detail. I guess its sound to say that it may be used as a piece of evidence to further narrow down the target, but its completely disingenuous to claim that it's sound evidence.
Like are you joking?
Saying TouchID validates fingerprinting is like saying MRI machines validate psychics.
It seems you two have a different idea of what is being discussed. The underlying fundamentals of finger prints are solid. The part that can make them ineffective is when they are using insufficiently tested tools or matching on too few points. And of course misrepresentating what a match actually means and how it pertains to the case.
What exactly are these underlying 'fundamentals'? Nobody even knows if everybody has unique fingerprints, it's just an assumption.
Beyond that, it's not all that relevant since as you mentioned how the matching is done is largely subjective, and it depends a lot on the number of "points" used and who does the matching, and that should really bring the accuracy even more into question. What exactly is the statistical probability of an incorrect match based on the number of points used? Good luck answering that question.
IMO the situation is made a lot worse by the fact that the public generally assumes fingerprinting to be extremely accurate (or 100% accurate).
How then do you explain the success of Touch ID on Apple devices and the equivalent systems on other phones and tablets?
Random people can't just walk up and unlock a Touch ID device with their fingerprint, which suggests that fingerprints are in fact a very good way to tell people apart.
Yes, it is possible to make a fake fingerprint that Touch ID will not be able to distinguish from my fingerprint. But that fake fingerprint will only unlock my device, not your device, so it is still correctly distinguishing our fingerprints.