And I've already GOT my music. Fair use doctrine tells me I should not have to pay to snip a piece of it into a ringtone.
Usually a provider needs to get permission from the rights holder to use the song in a significantly different way than what has already been granted. Since only a percentage of the songs are available for this, I suspect this is the case with Apple.
For each song that Apple wants to let you turn into a ringtone, Apple probably has to get special permission from the owners and then pay them each time someone does this.