Probably the difference between having Apple replace it and doing it yourself with a $20 battery you bought off of the Internet. Well, that and a few hundred bucks.

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.