When my retina-display macbook pro battery swelled, they replaced the entire top case. It appears to be policy that the top case and battery are one piece.
I can see why they wouldn't want the techs doing that in the store, one of the failure modes if you screw it up is a lithium fire. Also, if the battery had started to swell up it may have bent the case. Apple doesn't leave much room for expansion with their batteries anymore.
Got the $80 battery from ifixit and replaced the battery relatively easily. It did have me pour generous amounts for aceton (nail polish remover) into my MacBook to unglue the battery though, was happy that everything still worked after.