Hmmm, my lad just tried to update with his to iOS 7 and it wouldn't activate it becuase his phone wasn't registered. Weird, let me do it fine and I've never registered.
Guess it can only be one of 2 things
1: they have switched on the checks again
2: he did update his phone to the latest proper version over wifi. I always update via iTunes.
Either way, I had to put it in dfu mode for him to restore back to iOS 6.14