Earlier quoted context omitted.
https://www.theverge.com/2017/8/17/16161758/ios-11-touch-id-... is how you work around that issue. Keep in mind, you don't need to refuse to TouchID/FaceID forever, just for the timeout (which I do wish was configurable -- I think one hour is reasonable).
Here's the sequence of events: 1. You're walking, with your phone in your pocket. 2. Suddenly, a cop accosts you. You don't have time to react. 3. They detain and restrain you (with handcuffs or otherwise) 4. They pat you down and find your phone 5. They hold up your phone to your face to unlock it I know that people who've never been detained or interacted much with cops think that this is a completely unlikely situ…
FaceID / TouchID are a "convenience" to be used when you are comfortable with your surroundings. Can you be caught off guard even when you are paranoid? Of course, nothing is ever guaranteed in life, except death and taxes as the saying goes!