John Gruber thinks it’s because BBY is losing CD sales to iTunes, but I’m not convinced that they’ve ever seen significant profit from CDs. The margin’s too thin. They’re too busy selling extended warranties and gold cables.
CD sales got people into the stores, which helped overall sales, but that’s not going to improve by owning Napster.
So I’m not really sure why it was in their best interest to buy a failing music distributor.