This is the right way to go about this sort of thing. It doesn't say "we're gonna break it, na-na-na", it's just a simple statement of fact that you takes your chances.
I run across this all the time. I develop integration solutions, including SOE images and other things. And when my solutions are mis-used, deliberately misconfigured or outright ripped off, when someone comes to me complaining, for example, that they lost all their data when they installed my image, enabled and logged in as root (true story), well.... sorry, but tough luck.
I don't develop with the intent to maliciously outright block or destroy or anything, but at the same time, I have no obligation to develop with the rogue element in mind - how can I when I can't really predict what they're going to do? The only thing I can do is develop with the genuine, true intention of what my product is supposed to do/achieve, and, as Apple has done here, state that any use outside its original intention is neither supported nor my problem.
I'm inclined to give Apple the benefit of the doubt here and take them at what they say (ie: the opposite of "if in doubt, assume the worst of someone" which seems to be the norm here), that they're probably not going to go out of their way to block Palm devices, but at the same time, if there's an issue, it's not their problem and they're not going to go out of their way to fix it.