Unauthorized account activity

Hi, was wondering if this is legal or not. I had a bank account open with Farmer's Bank and someone who made a payment to me into that bank account later used the same information to charge over $300.00 on the account and the bank allowed this to go through without my authorization or knowledge of it. I found out the next day and immediately went to the bank to find out what happened and told them that I had no knowledge of it at all, nor did I authorize any charges to the account. They refused to do anything about it, and said that I would have to pay them the full amount plus any additional charges.
So is this legal?