They offer free PDFs if you buy it from them via subscriptions. Offering discounts for buyers from other sellers would dig into that exclusive benefit.
The PDF is only $15, I brought the book off Amazon and the PDF off Paizo which worked out cheaper then buying the CRB directly from Paizo due to international shipping.
There is a way to get a discount on past PDFs. If you start up four or more qualifying subscriptions, then as soon as the fourth one ships its first product to you, you will have Paizo Advantage. Paizo Advantage gives you a discount on certain purchases, including all PDFs from Paizo.