Perhaps Apple is afraid that someone will try to steal their icons. Being that they don't like to take chances with that kind of thing, they have filed trademarks for their App icons on the iDevices.
For example, the trademark for their Camera icon, which is now filed, describes icon with the lens that comes pre-installed on every iPhone.
Here are some interesting pieces of information on the Camera icon:
"The color(s) black, blue and silver is/are claimed as a feature of the mark."
"The mark consists of a rectangle with rounded corners depicting a stylized camera lens."
It's filed at a "944 x 944" pixel count. It's said that the icons on the iPhone are displayed at 57×57 and the iPad displays them at 72×72. This can mean two things: 1) They scale the image down to fit the format. 2) The iPhone 4G is going to be REALLY big.
It's application is for, "Computer software for taking, recording, editing, sharing, and viewing images and video content."
Apple has also filed trademarks for the iDisk App and the MobileMe Gallery App. Seems they felt the need to make sure no other OS uses their icons.