I'm not saying thats for sure, i'm saying thats my guess.
Or, they may have some drm protection somehow built into the mp3s that they only work fine on the original computer that they were ripped on... I know wma's have built in drm features, but I didn't think mp3's did, so I wouldn't think that the original comp that it was ripped on would be fine. I think they'd probably tell u to subscribe to the MP3 version of their service instead of their CD service.