Wasn't there an initial report out there that said Rock Band 2 would have over 120 songs on disc? Then more recently they've been saying that there would be 80 songs? Maybe the discrepincy comes from the fact that there will be 80 new Rock Band 2 tracks on disc but that they'll also have all of the songs from Rock Band 1 on disc too. Just a thought.
In either case, I think that's the only way HMX would release Rock Band 1 songs on Rock Band 2. Otherwise, if they gave Rock Band 1 DLC for free to Rock Band 2 owners, people could just rent Rock Band 1, get the songs as DLC for free, and use them with Rock Band 2. I'm sure that's a problem HMX wants to avoid. So, having Rock Band 1 songs on disc with Rock Band 2 seems like the only possible way they'd do it.