It's in the T+Cs you agreed to when you signed up.
Of course we all read every word of the pages and pages of micro-print before we click the 'agree' at the bottom of every bit of software we use don't we.
I too used more than my 10Gb/month (that's why I initially signed onto what I though was a CONTRACT for 40Gb/month) so I took up Sky's offer of Unlimited free for 6 months as they would have put me on it soon anyway.
TBH the bandwidth/usage meter has been on Sky's site for a while now and there have been loads of posts about it too.
Sky were a bit sneaky putting the info out as junk mail with the Sky mag but I suppose they did give us a chance to jump before we were pushed.