All ISPs have a fair usage policy somewhere in their contracts, just often not worded as such. IDNet terms and conditions state:
From their T&Cs
We shall be entitled to terminate the Agreement without the need to give any notice
in writing to you upon the occurrence of any one or more of the following events: if we in our opinion believe that termination of the Service is necessary to protect our goodwill or reputation and/or of our services or products (or necessary to protect the service provided by us to other users)
; if we in our opinion believe that you are in breach of the provisions of Clause 3 or Clause 4 of these Conditions; the receipt by us of any complaint, letter before action, or proceedings by any third party in relation to the manner of your use of the Service, or the content of any material published by you through the use of the Service.
So that's a fair usage clause just written in another way. How or when they would use that clause I don't know and sometimes having an actual usage amount specified is the fairest thing.
One thing we should accept that just because something advertises "Unlimited" doesn't actually translate to unlimited no matter what