My understanding is that it stemmed from them being able to compel you to take your fingerprints. In the old days you'd put your fingers in ink then put them on paper so they could record your fingerprint. I guess a judge ruled that putting your thumb on your phone isn't substantially different than that.

That said, you shouldn't ever rely solely on biometrics. In addition to the legal reason, an adversary could also unlock your phone if you were unconscious / etc using your thumb.