Now I get the argument that making your product a "closed" system like this is counterproductive. If you discourage third parties from making hardware/software that's interoperable with yours, you will eventually get out-innovated by a competing platform that welcomes them.
But in the mobile music player space, Apple doesn't have any competitors on the horizon. That's why they can take their customers' lunch money and leave them clawing their eyes out for more. And even to the extent that someday there's a Zune Shuffle to worry about, the battle is not going to be won or lost over whose players are compatible with the greatest variety of headphones.
So why should they leave money on the table, is my take.