My guess is
The warranty is from the shock company and they cover the product only. Unless it was installed improperly the installer should not have to eat the extra labor. Of course with pro rate its different. I beleive they sell batteries that will last 4 years under a 5 year warranty so you come back to them and get a new one and they knock 20% off as pro rated. (hell wait for a sale you can even do better. But know you have a new 5 year battery you feel good they honored the warranty and in 4 more years you come back and get a battery from them again.
"Take it up with my butt, cuz he's the only one that gives a crap"
Carter Pewterschmidt