I get relatively annoyed about the facile pedantry of this argument, but this is exactly the poster case for it, so here goes: fingerprints are usernames, not passwords.
The lock in your bathroom isn't meant to secure the bathroom. It's just a way to ensure that people get the message, "please don't enter".
A fingerprint on a phone is a way of saying the same thing. This phone isn't meant for common use, please don't enter.