Again, you don't have to buy it, it comes with it for crying out loud, and its not like they have a ton of them laying around. These lenses are not exactly mass produced, and so the stock of a part that 90%+ of users will NEVER need to buy probably isn't going to be very big, ergo the price is high.
For instance, I have had a 300 2.8 for over two years now, the amount of times I have had to buy a new hood? 0. The amount of times I think I will ever need to buy a new hood? 0. Therefore barring very unusual circumstances, I will have to spend $0 more on a new hood for as long as I own the lens.
As I said, these things are built to take abuse, and have a large rubber ring around the end which could take any small bumps or impacts with no effort. In order to break that hood you would have to do so much damage that you probably would risk breaking the lens itself aswell. The only other way to need to have to buy a need hood would be to loose it, which seeing as it bolts on to the lens with a thumb screw, could pretty much not happen.