When a bank waives your monthly fees, it's not just being nice. The requirements you must follow help the bank keep its costs low or raise its revenue. One way a bank raises revenue is by loaning money, which requires cash. And in order to have cash, the bank can require its customers maintain minimum balances in their accounts.
A minimum balance is an average taken over a certain time, typically a month or a statement period. If you have an account that requires a minimum balance, the agreement with the bank explains how the balance is calculated and what fees you'll be charged pay if you fall below it [source: FDIC]. That might not seem fair, especially when you're not used to paying any fees for your account, or if the bank didn't notify you to resolve the situation first. However, the bank is simply charging a fee you agreed to pay, and it has made you the party responsible for keeping track of your balance.