My question involves an auto loan or repossession in the State of: Wi

We have an Auto Loan with Wachoivia Dealer services. We have always paid them by check, never electronically. We are late on a car payment this month so they decide "hey we have all their info on the check" and "we'll just put through a bank transfer". So they put through an ACH transaction on our bank account from the info they got off the check. We NEVER authorized such. We are now overdrawn by $500.
I have read about this and fully understand that once you authorize an ACH transaction it is difficult to establish that further transactions are not authorized and it gets sticky. But when we NEVER authorized one, just wrote them a check (that they took the account and routing number off of) How on earth is that legal?

Also when we call the bank and tell them of this unauthorized transaction, the bank says we should contact Wachovia Dealer and ask them to fix their mistake. What?! Wachovia Dealer knew we didn't authorize it, it was no mistake, it was a scumbag move to get the payment without our permission.
If someone broke into your house and stole your TV would the police say "call the thief and ask them to give it back"? um, no"