It's planned obsolescence, there's a good documentary on it:
http://www.youtube.com/watch?v=Y1xt4nEvipg
Apple was actually sued over a similar thing with their iPod batteries. Now they offer a replacement service.
Of course the price to replace the battery vs the cost of a new updated version is so close just about everyone will buy the new product.
Plus once the battery fails, it's not going to be to long before the flash memory follows which would be another repair.