On the 360 and PS3, songs are stored separate from normal DLC too. I think there's other things missing, but I'm not 100% sure. It does keep them out of RB1 (not a problem with the Wii).
Problem is, if they're handled like DLC now, and not later when RB3 comes out, you'll have numerous compatibility issues. Better to wait till then (especially with SDHC support allowing for larger song libraries) than to screw things up for later.