The most secure thing about a password is that generally, it's either very difficult (impossible I believe if you're in the US) to legally make you disclose it or enter it.
Whereas in most situations with law enforcement you can be very easily forced to unlock your device with fingerprint/face unlock. This isn't just with the big agencies like the FBI or the CIA, this is just police, or airport security. In many of these situations they just decide it's easier/safer to violate your privacy and force you to unlock your device for them. Sometimes they'll take your phone, copy the entire contents to their drive, do god knows what else, and give it back to you.
Thus, easy solution is just shut down your Apple devices and you can't be forced to unlock them because they simply won't do biometrics until you put in the password. Even in a place like Australia where we obviously don't have the fifth amendment, you still have to be dragged in front of a judge in order to be compelled to provide your password, and that protects you from routine nosy lazy law enforcement, airport security etc that have no legitimate reason to get into your devices. Their policy is to try to intimidate you into unlocking your device, and when you refuse they eventually just give up unless they actually have a serious reason.