Yes, it is. If I pay for a product, it's my prerogative to use it in any way I please. Copyright grants a monopoly on distribution, not on controlling post-purchase usage of the product.
It's one thing to say "we only officially support installation of this product on this list of devices, and can't guarantee it will work on other devices", but inserting artificial restrictions into the product that prevent people from even trying to do officially unsupported things with the product, at their own initiative and at their own risk, is something there really isn't any justification for.
> do I really want the pain of people using it on an Arduino?
What pain? How does it even concern you if people are installing your firmware onto an Arduino, provided that they have legitimate copies of it?
> Opening it up to all the devices dilutes the main thing it has going for it.
Obviously, not installing OSX on other devices is what dilutes the "main thing it has going for it" for those people who have chosen to install OSX on other devices. Are you seriously trying to tell people that their personal preferences are objectively wrong?
> Sometimes there is freedom in restriction.
Straight out of Orwell.