Actually the word "unlimited" is set to restrictions under their Terms of Service which is expected by every company that the customer read and therefore is automatically 'signed' when activating their service.
As it states, the user used the phone to tether which is not supported and therefore he abused.
If he used the phone, #1) He wouldn't have as ridiculous charges, #2) He would probably not download as many high bandwidth files to his phone, #3) The charges of abuse wouldn't be as bad as the one's he received. (He should be happy they are what they are and is ultimately fighting a losing battle)