Lithium batteries which are 'in a device' will often have protection circuits built in to protect the device in question from going unstable, hence why laptops, iPads etc can be carried in the hold.
If however they ship the battery on its own , in some, but not all cases, the battery doesn't have a protection mechanism so could run wild and explode. Bizarrely, if Amazon ship you a spare battery for a laptop, they won't tell Royal mail in the UK that its being shipped, but if you do a return item back to Amazon, Royal Mail will refuse to accept is because its a battery.
Portable recharge units for mobile phones must be in hand luggage and not in the hold for this reason.
Of course, the entire rationale is rubbish but it keeps the H&S folks happy.