Honestly, everyone is saying apple F'd us on purpose to make more money. And while I don't doubt that they love to make money off of accessories, I don't believe they did this on purpose.
As an engineer of portable devices, I can honestly say I probably would have done the same thing as Apple.
If you look first at the IPOD AUDIO vs STANDARD AUDIO, you'll notice the LEFT and RIGHT channels are reversed. I'm not sure why they did that but it's bunk. Something to keep in mind when using any standard audio cable as well.
Now look at the difference between STANDARD A/V and STANDARD AUDIO. LEFT and GROUND stay put on the same pin, but for some bizarro reason the standard is to put VIDEO where the RIGHT channel would be, and move the RIGHT channel to the new 4th pin.
Now what Apple did, is they said.. hey, we have to make our iPod work with standard headphones and audio cables, but since we have to make our own custom A/V cables.. let's just put the VIDEO output on the 4th pin. This makes it's easy for them because they don't need any extra circuitry to switch the VIDEO and LEFT signals. In fact, it's not a cheap or space saving solution to have to switch those signals.
So in my opinion, Apple just tried to save some money and PCB space and give us audio and video from the same jack.
If you look at standard a/v and audio cables, they are not interchangeable with each other. The reason why when you put headphones into a camcorder's A/V jack you only hear the LEFT channel, and you may get some static on the RIGHT channel because it's actually a VIDEO signal.
If Apple wanted to totally screw us, they would have swapped the GROUND and LEFT signals in their jack on the iPOD. This would render a standard cable useless, and require you to cut the wires and reconnect them in the proper order. However, this would also make all standard headphones and audio cables not work.. I think if they messed with it too much people would really look down on them with disgust.
It could be viewed as them just tweaking it to trap the average joe into purchasing their A/V cable, but honestly I think they just took the easy path in engineering the iPOD output circuitry.
Hope that was enlightening ;)