they are already written, no/zero/nada extra work has gone into making these books, no materials have been consumed, they have been free to produce they could sell 1million copies and only incur amazons costs.
This isn't about entitlement, get to know me before you start talking like that.
From a business model point of view, selling them for £2-3 each would a) get more fans to buy them again, b) introduce new fans (who might otherwise buy secondhand at zero benefit to the publishers) c) those new fans will then also go out and buy/watch the films etc.
Benefit to both the publisher and the customer is positive.