As someone else pointed out, the aPencil is designed to work with the digitizer/screen in the iPadPro.
So, if the same screen tech is used in another device, then the aPencil should work with that, too.
Curious to see if it does anything on the iPhone 6S. (different tech, so probably not)
Maybe you will see that Apple can do software or firmware changes for existing devices (or new devices, such as a more "touchy" MagicPad) to allow the aPencil to work.
Right now, it's an accessory for the 'Pro, as the 'Pro is the only device with the tech that the aPencil can use.
No one is saying they are excluding 3rd party styluses. What I am saying is the Apple Pencil won't work with other iPads. Only the iPad Pro.
"iPad Pro knows whether you’re using your finger or Apple Pencil. When iPad Pro senses Apple Pencil, the subsystem scans its signal at an astounding 240 times per second, giving it twice the data points it normally collects with your finger. This data, combined with Apple‑designed software, means that there’s only milliseconds between the image you have in your mind and the one you see on the http://www.apple.com/apple-pencil/?cid=wwa-us-kwg-ipad."
I wonder if the aPencil might work as a stylus to some degree on any other pad, but won't have the various stroke changes and pressure effects. That requires the higher speed scanning that the iPad Pro offers when the aPencil is being used. Without those features with high-speed response, it's just another stylus. If a third-party stylus appears that can access the sensors in the iPad Pro screen, then that should be a good thing - assuming similar performance to the aPencil/iPad Pro pairing.