Rock Band has pushed cross-compatibility (music wise) as one of it's major selling points. The DLC and (most) songs from RB1 were portable to RB2, and the same with Lego Rock Band.
This doesn't apply to The Beatles Rock Band, unfortunately, but given there are enitrely new gameplay mechanics this isn't really surprising.
Guitar Hero seems to be sticking to a much more common franchise approach, which generally just leaves a bad taste in my mouth.