They don't have a monopoly though. They're dominant in the market, but that's not the same thing. There are other places which sell music - Amazon springs to mind. There are also many ways that various mp3 players sync music. I use rsync myself, because my mp3 player presents itself as a sensible filesystem.
I think there's dodgy ground because they are deliberately locking out competitor's products in a different market. Ethically, that may be wrong, but it's not monopoly abuse because (a) they don't have a monopoly in either market and (b) this practice isn't done to encourage ipod sales. Apple are all about controlling the experience, and allowing other players to sync with itunes affects their brand and image. They're not trying to suppress rival players, they're just protecting the brand.
Is it anti-competitive? Probably. I'd argue that whenever any vendor engages in some sort of lock-in, it's anti-competitive though. Doesn't automatically follow that it's illegal.
Apple have nearly 90% of the portable music player market with the ipod (source) and due to the itunes lock-in, 57% of the digital download market (source)
The anticompetative nature of the Ipod/Itunes is far more anticompetative than anything microsoft have done in the last 15 years (it would be the equivilent of preventing any alternative browser running on windows for example) and should be addressed, because apple not only have a monopoly (the EU definition, incidentally, is over 30% market share) but they are abusing it in ways that microsoft never have, nor would ever get away with.
Perhaps the problem is Apple's pockets aren't really deep enough, further proving that monopoly action is not to benefit consumers...