Zero of my 2,400 songs are available for ringtones. If I buy a CD, I should have the right to take 20 seconds from any song and use it as my ringtone. Not cool Apple. Not cool.
I know everyone blames Apple, but you should try to read the convoluted music licensing laws before you're so quick to jump. From what I understand, you are NOT legally within your rights to create a ringtone from a song you already bought -- and lets face facts, a lot of people here already admit they didn't buy their MP3s to begin with -- but even if you did, your "personal use license" (which is what you get when you buy a song or CD) doesn't include making that song (or a song from that CD) into a ringtone.
Nearly ALL cellphone providers sell ringtones, and though they seemingly make it easy for you to transfer your "free" MP3 ringtones you created yourself, that doesn't make it legal... and because those same cellphone providers aren't music retailers (like Apple is), they probably aren't under the magnifying glass of music publishers.
In other words, because Apple is THE largest online digital music retailer, they have to play the game. If they make it easy for you to create ringtones out of the songs you already have (not to mention all the ones you have illegally) they are not following the licensing laws... and NO music publisher is going to go along with that. Apple is not going to risk it's relationships with its providers of digital music. That's just bad businesss.
For the ones who are determined to get around copyright laws and make ringtones anyway, we already know there are tools to do it... but don't expect Apple to make it easy for you. They just can't. You can't expect the largest retailer of online digital music to help it's consumers break the law.
The rights to a song as a ringtone are separate from the rights to the song as a music track. Blame the RIAA or music publishers.