Why can't I buy the defib with a large battery in it?

I'm sure there are bigger issues with the game, but this is my pet peeve. I can feed any magazine I want into my rifles, why is this not the same case with batteries? It saves me 1 inventory slot to use the big battery in!

Worst of all, if I reload the battery to a large one before leaving the base, the next time I visit the shop and rebuy my loadout, I get an error saying the rebuying failed.

And god forbid I dare to actually use the slot the bigger battery gave me! Then, after you rebuy, you have to drop an item from your inventory, buy the large battery, reload it, and pick the item back up from the ground. Extremely frustrating!