Again, it's Apple's store. You can buy through the web browser, it'll take you an extra 20 seconds, tops. It was a link designed to sell books without giving a cut to the vendor. If I made a trial version of a game, tried to put it on Steam but didn't allow the full version to be bought there and instead included a link to buy it exclusively from my website, it would get thrown off.
I think the cut they want is probably too large, but this is standard practice and really no different to anything ton of other digital content vendors do. The only difference is that Apple allowed it for a while, hence the uproar now that it's changing.
To buy, if you do not have an account, you need to actually go and create an account. Now, have a look at the updated app for Amazon, Nook, Kobo and others and see if you can find any information on how to create an account or where to go to create one. Can't find it? Funny that! The lovely guidline/rule 11.13 in the App Store Guidelines made mention of removing the options to purchase content form within the app, but it is also being used to force developers to remove any mention of their website (you cannot even say "our website" that'll get you rejected) and any information regarding how / where the user can go to create a new account.
Apple is forcing people to use IAP, if you don't, you are not allowed to inform the user in any way, shape, or form, of where your website is, or how to get a new account there. All in the name that it's allowing users to purchase content outside of the app.
The major key points here are that most ebook sellers are vendors and NOT publishers. A massive distinction. When it comes to a book sale made by a publisher then they get 100%, using IAP they would give 30% to Apple, so still have a nice chunk left. When it comes to the vendor, the publisher gets 70% the vendor 30%. Throw in IAP and that 30% is eaten up by what is JUST a credit card processing fee. The vendor still has to manage and store the content being processed, and they are getting 0% for doing so.
Most, if not all of the ebook apps are free, so it's not as if the vendors make money on the sale of an app, and in that case they'd be charged 60% by Apple, 30% for the sale of the app, and 30% if IAP is used.
The other key point is that IAP is not currently set up to cope with a very, very large catalog, and ebook vendors can have, at the very least 50,000 plus items. From what is known, IAP has an upper limit of about 5000 items. Additionally, there is no tool to mass create SKU's, so you'd have to manually add all those items. The pricing structure is fixed, and would not align with publishers prices. You'd need to go to the publisher and ask them to change their prices (they set them, not the vendor) to match the fixed pricing structure of IAP, and I doubt they would do that.
There is also the cost of implementing IAP, and who would really want to implement a system that results in their app still making zero money?
Apple knows exactly what it is doing here. It wants iBooks to be the only ebook app on iOS.