The reason you have to delete is that iBooks are a packaged product - meaning that there isn't a way to do an incremental update. So if the book is updated, you technically delete then redownload the book. This is a limitation of Apples iBook system - not corporate conspiracy from GW to get you to repurchase a book. As someone who uses iBooks quite a bit, I've not had an issue with this process.
To add a bit more - this process is identical to how apps work in the iTunes app store. If you have an iPhone or iPad and have been using the app store (and getting upgrades for your apps - which are deleted and redownloaded also) then this is nothing new.
Last edited by HadronFlux; 06-25-12 at 02:58 PM.