Since the beginning of fingerprint unlocking of phones, but brought into prominence by Apple's Touch ID, there has been a battle over whether or not law enforcement has the right to compel you to unlock your device using biometrics. It has long been ruled that unlocking a device via PIN was a violation of the 5th Amendment because it requires a person to divulge private and confidential information, which is tantamount to testimony.
On the other hand, biometric sensors have been thought to fall under a different set of rules which apply to things like requiring a person to take a urine test for alcohol. Since no information is required to be divulged to unlock the device, and the process of unlocking the device is in no way invasive, the legal standing has been that law enforcement had the right to compel a person to place their finger on a sensor, or to look into the camera of a phone.
The legal standing has been challenged on multiple occasions with differing results. However, the prevailing precedent has held that law enforcement can compel. That precedent has been changed this week, as US District Court for the Northern District of California magistrate judge Kandis Westmore ruled in opposition, closing any jurisdictional question on the law. In her ruling she stated,
If a person cannot be compelled to provide a passcode because it is a testimonial communication, a person cannot be compelled to provide one's finger, thumb, iris, face, or other biometric feature to unlock that same device.
This is obviously far from the last time we will hear about this scenario. There is little doubt that something of this nature will make its way to the Supreme Court for a final ruling, but for now, this is the law of the land, and it is good for all of us. Previously, the only real hope we had in protecting our data from the prying eyes of law enforcement was a feature Apple introduced with a completely different purpose.