There is no monopoly leveraging here. Apple is a hardware company and made an MP3 player that people wanted. That device is useless without an app to interface with it, so to actually use the iPod/iPhone, they wrote iTunes. iTunes was written to manage the iPod, not anything else, the iPod. iTunes exists to make the iPod/iPhone easy to use.
Actually that isn't true. iTunes came out before the iPod and originally supported a number of other MP3 players.
http://support.apple.com/kb/HT2172
In some ways, I think it would be beneficial to Apple to encourage 3rd party sync plugins that integrate fully into iTunes for Mac OS X. Unlike iTunes for Windows, iTunes for the Mac was intended to provide Mac with a full featured media player.
It is really to Apple's advantage to have support for devices other than the iPod and iPhone. Why? Well, let's say my company requires me to use a Blackberry. If I go to the Apple Store and they say, "hey, yeah, that'll work right out of the box with Mac OS X's synchronization tools" that is an instant bonus. One more reason a Mac "just works" (unlike Windows).
If you are Apple, you don't want someone to feel like they must dump all of their accessories just because they switched from Windows to Mac. For that matter, you don't want a phone (or media player) to ever push someone to switch to Windows.
Remember, Windows now has a centralized sync facility. Apple's iSync is growing rusty and outdated. I don't think unless Apple wants to expand and offer a wider selection of phones it is safe to act as if OS X only need focus on support for one phone.
(I say this as an iPhone and now iPhone 3GS user.)
Here's what Apple should do: they should insist Palm stop spoofing. But, they should release the API's necessary to allow developers to create synchronization drivers that integrate into iTunes. Maybe dump iSync since it is pretty much ignored and just add its remaining functionality into iTunes.
-Tim