While it's definitely the biggest incentive, there's also a customer-focus piece. I used to do third-party repairs for these devices and I can't tell you the nu

While it's definitely the biggest incentive, there's also a customer-focus piece. I used to do third-party repairs for these devices and I can't tell you the number of people that would get mad at me and/or Apple because they jailbroke their phone and then did something to brick it. They had someone else jailbreak it for them, so they had no understanding of what had actually been done, and then they attempted to run updates that broke them. When people expect support, it makes sense to me that Apple would want to prevent those types of situations from occurring when they're the ones that have to deal with people who don't know or understand what they've done.