John Gruber via Daring Fireball
It’s the difference between “What’s the best we can do within the constraints of the current ePub spec?” versus “What’s the best we can do given the constraints of our engineering talent?” - the difference between going as fast as the W3C standards body permits versus going as fast as Apple is capable.
This design trade off is just like Apple’s 30 pin dock connector. Many try and argue that a mini or micro USB port that is supported throughout the industry is a better choice that an Apple only port. I see the other side to that argument. The 30 pin dock connector offers not only power and data, but video, audio out, audio in, FireWire, midi, ect. ect. Apple made the choice ~8 years ago to push forward with their own port with the features they wanted rather than wait for a standard port to be adopted. I think that choice has turned out ok.