Apple doesn't allow the playback of WMA because then the iTune Music Store wouldn't drive iPod sales as effectively.
I am sure that the chip can decode all three codecs, but it would more than likely require a firmware hack to get the iPod to play WMAs, not to mention some new software to load the files in the way the iPod likes them.
Of course there's the matter of the royalties due to Microsoft if Apple enabled WMA.
Based on 1st Quater 2004 sales of 733,000 iPods, that would be a nice cheque for more than $100,000 in that quarter alone.
Mercifully that figure is kept down because Microsoft caps the software licenses at $20,000.00 per year (I license for the iPod's software, I for enabling playback in iTunes for Macintosh; it's free to enable it for any Windows software so enabling WMA 2-channel audio in iTunes for Windows would cost nothing).
The cap on enabling it in hardware doesn't kick in 'till you've given them $ 400,000.00 in a given year.