What is the need for mp3 rate restrictions?

I am getting this error when trying to upload most of audiobooks I have:
Field “audio”: The audio sample rate must be either 11025, 22050 or 44100 Hz. The sample rate of the audio you uploaded was 24000.

What is the need for such restrictions? Why won’t you just let upload mp3s with any rate within these 11025-44100 Hz?

Not all software or mp3 players support all ranges of sample rates. In the interest of the users downloading the mp3 files, we decided to add these limitations to abide by the standards.

Here’s a link to a few applications that can help make the conversion process easier: http://bit.ly/wO5Jan