Since technologically, this would be very easy for Apple to do, I can only assume they do not have a feature to lock certain applications, as a marketing ploy. Specifically, they want to encourage everyone in every household to have their own iOS device. It is the same reason that they have until now not offered a multiple account login for one iOS device. Technologically, this is all simple stuff, but they don't do it because it doesn't comport with their marketing goals.