Licensing new songs for a re-release would take a lot of time. Charting the songs for play is another, and that takes a lot of time too.One possible explanation I've heard for this bullcrap is that the licenses EA hold for the music will expire on that date, or something like that.
Still, leave it to EA to pull any sort of dirty business practice as not allowing people to continue using a product they paid for. People probably did not buy the download expecting an expiration date on the playability. The support and the servers perhaps, but not the actual ability to play it.
It could be they really were going to pull the plug, but the outcry made them reconsider the action and call the message "an error".
If that was true then they could re-release it with music that they would have no problems using.
It wouldn't be worth it as it would be hypocritical of Harmonix to release a game with a bundle of songs whilst they use a perfect DLC strategy for the console game. Also, the market's been gone for a while and it won't be as popular as 4 years ago.