Update: Harmonix has responded via the following tweet from John Drake
Evidently your purchases aren't safe in the eyes of EA -- even if they're physically located on your device! RockBandAides is reporting that allegedly after May 31st, the Rock Band iOS App by EA will "no longer be playable". Many people are speculating that this is due to short-sighted licensing deals that are set to expire, and the DRM nature of the app itself, which could deny access.
Come to think of it, in the digital era, licensing deals are pretty insanely complicated. Just look at Hulu: even though it's technically the same service, there are different licensing contracts for web, mobile, and media hub streaming. Whatever the case, if this is true, it's going to suck for everyone that bought boat loads of DLC for the game.
We've reached out to EA/Harmonix for comment.
Rock Band iOS App “No Longer Playable” After May 31 [RockBandAide]