Harmonix's licence to use music from The Beatles in Rock Band is about to expire, and on their blog the studio says that they won't renew it. That means that starting May 5, all DLC songs will be removed from the store. If you've purchased the songs before that, you will be able to use them in the future.
Music by The Beatles was originally added in The Beatles: Rock Band, which launched back in 2009. If you're a fan and want to continue to play their biggest hits (and let's face it, there was a few), now would be a good time to make sure you can do that in the future as well.