Well, most ball pythons aren't eating mice for very long, anyhow. They're generally on rats by age 1. It's much more convenient to buy frozen rats in bulk online (and a big savings in cash, too). There's always a chance that the rodent will bite your snake, and sometimes these wounds can become infected, or are in a horrible location (like in the eye!) You don't want that to happen to your pet.
I don't recommend switching a ball python that is perfectly happy eating frozen, over to eating live.
Freezing has nothing to do with bacteria, it's about reducing parasite risks (freezing for several months kills most parasites), and about safety for the snake.