Its actually both. Blindly saying Apple is doing it for quality alone isn't right either, thats just defending Apple.
Look at cars, battery is very important there. There is a need to very have good quality batteries but still owners can change it. Car companies forcing owners to buy from them definitely involves money. Forcing simply isn't right.
The same can be said about proprietary ports like the one Apple uses. Each accessory is overpriced and users have no choice than to buy it from them. It has its advantages but MONEY is involved.
I still disagree. They want to cram as much battery life into the phone as they can, and this is the way to do it. Not only that but where would they put the battery door? Have you ever taken an iPhone apart? There just simply isn't room in there for anything else including a door mechanism. Its mostly all battery.