There is a huge difference from a small business in a competitive marketplace refusing to offer a product, and a multinational corporation in a market with limited competition. In the former case, refusing to offer a product isn't an issue. In other words, the "they own the store argument" is valid. In the latter case, refusing to offer a product is on par with denying a product access to the market.

Now it isn't quite that bad as things stand today. You can access other book stores, even using Apple's platform. But what happens when Apple decides that the Kindle, Kobo, and other such apps are competitors and they would be better off without those competitors? Worse yet, what happens if there is consolidation in the ebook market and there is only one or two major vendors to choose from?

It is probably okay for Apple to restrict the products available today, because there is a moderately competitive market out there. Yet I also believe that they have to be watched very carefully since it is very easy for this to develop into a situation where anti-trust regulation will need to be enforced.