A rear fingerprint sensor may trigger accidental purchases.

A rear fingerprint sensor on phone may trigger accidental purchases.

When we hold the phone, we may rest our index finger on the rear fingerprint sensor. In such case, whenever the UI prompts for confirmation of purchase via fingerprint, the confirmation is instantly finished even without user aware of that, due to the always there index finger.

So a rear fingerprint sensor may cause accidental purchases that users may not want.