Why is the iPad's iBooks App a Download?

With the release of the iPad right around the corner, there are still a lot of unanswered questions about the device itself. Yesterday evening, a friend and I were discussing some of the rumors that are surrounding the iPad. At some point in the discussion, he asked me if I had any idea why Apple made the the iPad's iBook application a download from the App store. When he said this, I had no idea what he was talking about, but when I went to the iPad page, this was stated very clearly.

The iBooks app is a great new way to read and buy books.1 Download the free app from the App Store and buy everything from classics to best sellers from the built-in iBookstore. Once you’ve bought a book, it’s displayed on your Bookshelf. Just tap it to start reading. The high-resolution, LED-backlit screen displays everything in sharp, rich color, so it’s easy to read, even in low light.

I couldn't understand why one of the iPad's main features would not be preloaded on the device itself. But after a few minutes of thinking, I was able to come up with a possible answer to the question. -- If you go into the App Store today, you will find several E-Book applications available for the current iPhone, which is exactly why Apple cannot preload the iBook App on the iPad.

As per Apple's current SDK agreement, developers cannot develop an App that competes directly with an Apple application that is standard on the device. If Apple preloaded the iBook App on the iPad, they would be forced to pull all of the other E-Book readers in the App store as per their own contractual agreement; making the iBook application "opt in" avoids this issue. This is purely speculation on my part, but I think this is a plausible answer to the question.  

Why do you think the iPad App is not included with the iPad?