A rooted phone is more capable of modifying the banking app itself and has 'freer reign' over the APIs that the app uses to interact with the bank.

Whereas previously the app displays a 'whitelisted' set of UI options to the user, the rooted user could use employee only methods. Somewhere or other every bank has methods that set balances on accounts.

To be honest a law like this makes security by the extremely modest obscurity of not having an "increase your balance" button on the app UI much more tempting.