The Beatles are huge. Discs on shelves in stores sell astronomically compared to DLC. HMX has said that the average RB owner has 4 songs as DLC. With 84 songs on the disc at a $60 price tag and 4 songs at a $8 (est.) price tag, where do you think the majority of the money is coming from? The people on these forums are generally the hardcore fans who buy lots of DLC (WARNING: blanket statement). There are hundreds of thousands of people who've NEVER bought a single song from the Music Store.
In short: Beatles game on the shelves will sell far more than Beatles DLC would. Especially if they make a pretty box, add some interesting new features, and do some sound marketing. HMX knows what they're doing here.