It's not arbitrary; they are trying to make the device more reliable, but again, I agree with you that they should give you the choice. At the very most, they should pop a warning that your device will be less stable if you save to removable memory, then allow you do to it if you wish. I'm just curious how much damage is done by prohibiting your preference. If you'd gain a third of a GB out of 16GB available, it's less of an issue than if you're losing 4GB.