I don't see the purpose of this.

A phone which authenticates me based on my face or fingerprint and provides a signed request to the terminal is far superior in privacy and security. I never see the point in moving biometric authentication beyond the user device.

Even in the event that the signing keys are compromised and are used without the biometric check you can have a chance to rotate those keys and etc.

Devices with known problems with their biometric verification could be prevented from registering a new key.

Never should a company like amazon have access to the biometric data itself.