Just tested. Downloaded a book on the phone that I'd bought on the iPad, and it gave me the "you have already purchased this book; press to downloaded it again" message you get from the App Store. If you go into the "purchases" tab it offers to redownload any books you've purchased elsewhere.
What the OP is seeing has to be a bug; after all iBooks does syncing between iPad and iPhone -- they can't expect you to repurchase each time.
"You have already purchased this but it isn't available for redownload. To purchase it again at full price, tap OK"
Given the exact phrasing of the parts I highlighted, I don't think it is a bug. I cannot see in the picture what book it is, but since this does not happen to you for your books, it could be something related to copyright/distribution.
What the OP is seeing has to be a bug; after all iBooks does syncing between iPad and iPhone -- they can't expect you to repurchase each time.