This us just my opinion, I admit that I know nothing about the law expect for the many laws that I have personally broken.
Unless they are covered in the terms of a contract then back-charges are not really all that legal. They become legal when both parties are willing to accept the compromise. In my opinion back charges are a form of informal arbitration which must be agreed to by both parties to be legal.
If they bill you and you accept the charges and pay them then it's admitting that you think you owe them money and thus it is then legal.
On the other hand you do have the right to argue the case as to why you should not pay, either in court or informally.
You still have the option to pay the charges, make a counter offer to pay some less amount or to refuse to pay alltogather. Whatever you do make sure that you get some sort of receipt showing that the matter is now settled ( signed ) and then things will be legal.