Why do banks charge a fee on "insufficient funds" when they know there is not enough money?

Yesterday J came upset saying that I had overdrawn my tiny lil Wachovia account. WHAT? I know I have not deposited in a while but I thought I had $75+ in it and it has been months since I wrote a check for a home improvement and I don't carry that debit card... Anyway... the short of it is that my paypal account is linked to that and when we bought our new stroller online (Maclaren Volo, LOVE IT!) and some personalized Disney shirts (for the kids surprise boxes) the funds came from the wrong account! Wachovia was charging me $57 in fees. The real gripe of it is that there were enough funds for the stroller but because it posted the same day as the shirts they bounced BOTH! To add insult to injury Paypal realized their error because and credited back the shirts and appropriately charged them to our family account. Basically that means I was getting double dinged when in fact there WAS enough money to cover paypals mistake! I called Wachovia and explained and the csr would not remove the fees. I explained that in my 22 years of having a checking account I have never bounced a check and that I WAS a valued customer. I have multiple checking, savings and credit cards with them and have for over 10years. All of my other accounts have more than enough funds (and overdraft, go figure!) so this was obviously an error. But nope he still would not fit it. I told him I would speak with my husband about closing every account we have and he wished me well!. After I hung up I called back and asked for a supervisor. I again explained I was being double charged when only one charge was short and that charge was backed out by the merchant when they realized their error. He checked my other account to see the merchant did in fact charge it after backing out the "bounced" charge. So he said he would credit the $57 back as it was obviously a merchant error. (Hello I was negative $45 after being charged $57... you do the math!) So yes at the end of the day I got my funds back and my lil personal account is back up to the $86 it started at but it was not because Wachovia cared 2 cent about customer service!