A fingerprint, like any piece of data, is handled at the lowest levels as a number. A number with some constraints, but a number.

By feeding numbers into the scanner instead of fingers, you can accomplish the same effect as feeding random strings into a password box. Further, it's also possible to take fingerprints through social engineering, or by getting at the database of a company that uses fingerprints as security. Five bucks says someone's already storing a bunch of fingerprint data as plaintext.