Regardless how this feels, this is required by the holder of the distribution rights for the song, not by Apple.
We don't own any of the music that we purchase or acquire by other legal means or by what is considered illegal means.
What we have purchased is a license only to use someone else's intellectual properly which includes restrictions.
With music ringtones acquired legally from other sources, you purchase a license to use the segment of the song (with the segment already determined for you as the ringtone). Unless you have purchased the entire song via a music CD or acquired legally by other means, you have purchased a segment of the song only to be used as a ringtone which is usually priced higher than Apple's combined method - for the entire song and for the ringtone with you getting to designate which segment of the song you want to use as the ringtone instead of this being designated for you. And with some, the license expires after a period of time requiring you to purchase the same ringtone again if you want to continue using it.
With Apple's method (which is approved by the those that own the distribution rights), you have access (usage rights) to the entire song, you can determine which segment of the song that you want to use as the ringtone, and the ringtone doesn't expire after a period of time - all for less than what most charge for just the ringtone.