I have to first ask: why?

The reason it's generating a PADSS-compliant password is for security.

Is there something preventing people from using the numbers on their keyboard?
What's your "minimum password length" set to in Admin->Configuration->Minimum Values?

It's REALLY BAD to allow people to use passwords with only letters, especially if they're short!

I realize you want it, but I'd really like to understand why.