This is very annoying for me too. I love the iBooks app, but I've bought far more books from Amazon instead (and a fair number of Nook books as well) just so I can read them on my laptops.
I don't even have a Kindle, but I've a ton of Kindle books and read them on my laptops as well as on iOS devices. I'm sure Amazon will forgive me skipping their hardware, since I could paper my livingroom walls with receipts from them for all kinds of stuff.
I do have and like a Nook tablet, but the fact that the books can also be read by a Nook reading app on other devices including a laptop was a VERY big factor in that purchase, which I made while I waited for Apple to bring the iPad mini. Now that Apple's mini is here I'm still buying books from the other two guys and using their apps to read them WHEREVER I LIKE.
Using a book on a laptop is much handier if I want to reference it while working out an adaptation of quilt design, for instance, in which case I'm likely also to have multiple other windows open in multiple other apps. "Don't try this yet on your tablet..." When I do something like that, I might be using several types of calculator, some inventory database pages, some spreadsheets, a browser tuned into a fabric supplier etc.-- as well as having a digital reader open to show me a quilt pattern in an ebook. Sure I can prop up an iPad next to the laptop with the ebook open in it, but it's more work, less convenient, and why should I do that when I can buy the Amazon or Nook book instead and use the Kindle or Nook reader application.
This topic makes me crazy when I let it get to me. Some say the no-laptop-iBook thing is because Apple can protect its DRM better that way or something. Well go for it, Apple, but I don't steal books, and the ones I have from B&N and Amazon are not DRM-free, I paid for them and I observe their terms and conditions, but I enjoy using them on my laptops sometimes, and appreciate being able to do so.
I rather suppose this complaint is like spitting into the wind. After all this time, I have come to believe that Apple just considers revenue from ebook sales pretty optional.