You authorized your computer to your iTunes account long, long ago. So why don't your iTunes Ringtones sync to your iPhone? Turns out that you're going to have to reauthorize your computer to allow it to use ringtones. Yes, not only do you have to sign a new terms of service to buy that ringtone but you also have to add a separate authorization.
Ah, there's nothing quite like the smell of RIAA in the morning.