Apple uses the device id to identify that a device is an iPod. That was, until the Pre appeared, a sure way to identify iPods. Then Apple figured out that someone (Palm) created a device that in violation of their USB device used a device id that made it look like an iPod. So suddenly Apple is confronted with the fact that there are iPods, and there are devices that are not iPods, but pretend to be iPods.
Apple is not allowed to use the vendor id to discriminate against a vendor. For example, if I bought a USB memory stick and the Mac rejected it because it has a vendor ID "Psystar" and Apple doesn't like Psystar, that would be wrong. But that isn't what Apple is doing. They are not saying "we don't let you connect iPods made by Palm". The are saying "there are devices that claim to be iPods but aren't. iTunes only syncs with iPods. We use whatever means we can to find out whether a device is an iPod or not. We know exactly which vendor id an iPod should have, so we reject anything else".
I'll just repeat it: It would be against their USB license for Apple to stop iTunes from working with iPods made by Palm. But they have the right to stop iTunes from working with devices that claim wrongly to be iPods, and they can use any legal means to detect whether a device is an iPod or not.