Youd think someone as presumably powerful as him would have made the necessary enchantments to not have his heart be able to be taken.
Regina did it to Henry. Why is this not like standard magical protection?
In some mythologies dragons do something like that, keeping their hearts outside of their bodies. If this dragon is as sneaky as he ought to be, he wouldn’t put up defenses that an enemy might, with time, be able to batter down. He’d let his enemy steal a decoy.