This is due to liability law. O2 is liable for the condition fo the phone they sold you, but if Apple take on the repair they adopt liability. Apple has a contract with O2 to repair the phone, not with you whom they don't have a contractual relationship with. They can't just waive that contractual relationship for every singe purchaser of Apple products from third parties world wide, it would open them up to a lot of risk. Suppose O2 screwed up warehousing and damaged a whole batch of iPhones somehow and then sold them to customers, Apple would end up liable for that. If you want a contractual relationship with Apple, just buy the phone from Apple.