Not that broad. Apple can check what they want all day, but I personally don't think it should limit other hardware to be used with iTunes to such a great extent. I don't know how they allow other devices to work with it "legally", but it is a free application that is included with all of their computers, is on many PCs, and essentially dominates the market because of the iPod/iPhone. Not that they can't do it, but that I don't think they should.
I don't think it's great what Palm is doing for the reasons you have mentioned, either.
I am guessing that other devices sync to it without a problem because they went through the proper channels and paid Apple the money needed to get the licensing rights. This is Apple's IP and if you want to play with it, you need to pay for it.
Palm obviously isn't taking that approach, they are just trying to pretend they are Apple and thus get a free ride. It is very possible that Apple just doesn't like them and doesn't want their stuff talking to their IP. Well, it is their IP and they can do with it what they want.
If you have a nice movie collection in your house, but you won't let me in to watch them, should I break in and then cry when you throw me back out? Nope. Also I shouldn't go around telling people that I can get them movies from your collection either.
Could Palm make a device that looks exactly like an iPod (or whatever device they are mimicking) and thus preventing Apple from detecting them? Sure they can, and it would work and there would be nothing Apple could do to stop them. IF they made it look EXACTLY like one of the Apple devices. But it looks like the device isn't perfect, otherwise there would be NO way for Apple to detect the difference.
It is in Apple's interest, as others have pointed out, to keep out intruders because it does start the slippery slope thing.. one does, it, so does everyone else, etc... before you know it Apple is forced to maintain compatibility with old devices just to keep them happy. And then it becomes a support nightmare.
Palm has a few choices.
a) Give up on iTunes syncing
b) Make their device IDENTICAL to the iPod (or whichever device they are using)
c) License the interface from Apple
d) Write their own software
I know from experience that b) is VERY hard to do; not impossible mind you, but hard. AMD did it to Intel a long time ago. Ensoniq did it to Creative Labs. (those are the two I know did it) Sometimes it is easier to just pay the money and make everyone happy -- but if Apple still says, "No" then their choices become more limited.