Not downloads...
"I could understand if it was due to DRM, but releasing the shows in MP3 format, surely negates any DRM incumbent with the AAC format?"
It's because it's not a download like the article suggests, but a streaming service. The point is that they're not downloads as the BBC can't provide downloads (save for what it podcasts) under it's rights agreements or indeed under it's service licence by the BBC Trust.
It's not a straight MP3 stream either, there's some jiggery pokery in the same way as the video streams to try and frustrate illegal downloaders.
A *download service* would need to have DRM on to enforce a time limit, but that isn't coming any time soon because Apple won't licence out Fairplay to anyone. And they'll only offer content through iTunes with DRM if it's costing money for Apple to take a cut of.
Such a download service would also require another large chunk of cash to be poured into the black hole that is the BBC Trust's Public Service Tests.