New / old battery

Asked local Apple techie shop for price for supply and fit a new battery in my three old MacBook and added I want the old one when replaced, back. Very reasonable price and they do a good job normally.

They cannot give me the old one (mine) back. It has to be returned securely to Apple. I am capable to dispose of it according to the Regulations. It was to go in a friends laptop which is older and not totally finished yet

I am curios to know if there is an ulterior reason for this ruling. Anybody know if it is the case that I cannot have it or of any other purposes for this.

I 'phoned a retail Apple source who said they would sell me a battery and 'No' they did not want the old one.

Curios.