Around about 2009 or so I had a mobile plan with Virgin that did that same trick (I think this was plugging your phone in to a computer as a modem as opposed to wireless hotspot but same thing anyway) and it was limited to 5 MEGABYTES after which they wanted 15pence per KILOBYTE I couldn't believe what I was reading. I never ran afoul of it because I checked this out first when buying the plan and made sure never to use that function but it just seemed literally unreal. I've been shocked at how much things cost before but that seemed more like a mistake or something, I just can't imagine they ever actually made anyone pay for that, the negative press would be too bad. It was not unheard of at the time for people to have excess charges on limited plans waived because it was a shock and they were unaware or unprepared for those charges accruing so the idea that someone might have checked emails, read a news article, checked Facebook and possibly a web video having not read the fine print and ended up with tens if not over a hundred GBP of charges just doesn't seem feasible. Really fucking crazy.