They justify it because it is genuine and not made from cheap materials by children in a sweat shop getting paid next to nothing <and breathe...>
Anyway, as Rick has pointed out others are available for a lesser amount and genuine ones can be found for a good price.
The main reason I wanted a genuine one was that I didn't trust the one that came with the lens and didn't want to spend over and over getting the wrong one time and time again. Oh, and the fact that when I use it it is holding onto £2k of camera and lens!