Would we need to commit to a new contract term? I mean, it's not like O2 are actually subsidising the iPhone, so why can't we just pay to upgrade our hardware and continue on the same contract as before?
I could understand why a phone company would demand a new contract if I were to buy a phone that they were subsidising, but in this case, they still get the (not inconsiderable) money for the phone, so why would they insist on a new contract?
Maybe 'because they can', and they're in the business of making money! Without any true competition, for the iPhone, O2 have the monopoly and can do what they want, and their contracts allow.