Because the Post is, to my knowledge, the first periodical without a paid website that Apple is allowing to sell app subscriptions.

FORBES: New York Post to Sell Subscriptions on the iPad

The limitations of the Kindle, in my view, are the limited selection of less-mainstream books, lack of backlight, expensive newspaper or magazine subscriptions and the inability to increase the size of graphs and other things embedded in texts.

FORBES: Intelligent Investing