I'm sure there is a reason beyond wanting to piss me off, but for the life of me I can't see an obvious one. And it is a real piss off. I've been doing a bit of marketing via Tumblr, plugging the fact that the book is free, and now I find that it's not free with the biggest e-book retailer.
Anyway, if anyone can be bothered, there is a 'Notify us of a cheaper price' facility on the book pages at Amazon. So it would be great if some people other than me notified them. You can use the links on the book page on this site to get the web site address needed for the notification. B&N is probably the best one to use. Click on the book cover shown here and it will take you there.
See! I try to be helpful - unlike some large corporates I could mention.
Interestingly enough - for the first month ever it looks like I will have more sales through B&N Nook than Amazon kindle. Could it be that the free promotion which they kindly support is bringing this about?