It occurs to me that the shrew mafia probably grew up much like the Sicilian mafia did originally: in the absence of any kind of functional police force, it became the de facto enforcer of law and order. Corrupt and partial law and order, but still better than total anarchy.

The big animals meanwhile are baffled that their anti-crime initiatives aren't working. Sometimes they don't even notice the openings through which the rodent criminals slipped in to rob the bank.