By making a device with built-in natural magnets designed to be attracted to an external leather case with metal is like making a PDA with exchangeable face plates but without an a replaceable battery.
First problem arises when the device comes within close proximity to other magnetically sensitive devices. Instead of being able to temporarily pull the cover off, you'll simply have to leave the whole ebook reader behind.
Second problem arises when the natural magnet loses magnetism, being irreplaceable, becomes just an extra dead weight inside the device
Sounds like a poor engineering decision to me.