It may not have been file sharing even if that is what the article stated. The users were taking their SIMS and placing them in USB dongles effectively tethering them. This could have been used for just browsing and mail. This machine that I have replied on has used 765 Mb in a few days and there is no file sharing going on. It is all email and browsing, maybe the odd iPlayer streaming but that is it and all completely legal.