As long as the Pre is capable of syncing with iTunes, you have a situation where an unsupported device is capable of affecting (through fraudulent means) the user's music, contacts, settings, and god knows what else -- all of which live in Apple's operating system, inside Apple's software.

If Palm fucks anything up and some fraction of Pre users manage to corrupt their data or their software (because Pre-iTunes interoperability is an ugly hack!), then those users are not—at least not all of them—going to shrug and say, 'Oh, well. I was using unsupported software and hardware, and I got burnt.' Enough of them are going to say, 'What the fuck!? iTunes/Apple/Mail/OS X sucks!' or they're going to call Applecare support and sit on the phone for half an hour while the poor schmuck tries to get their iTunes library back. That is what we call a bad experience, and Apple doesn't like that. If they control both sides of the equation, then they can make sure that doesn't happen.