As users we have NO IDEA how the licesing was done for all the songs. For the most part all we can gather is that the songs as DLC were done as DLC for "Rock Band" where the title of the game HAS to have Rock Band in it. as such games like Lego Rock Band, and Rock Band Blitz were able to access Rock Band DLC with little effort.

Beyond that songs might also be licensed specifically per console just as for use on '"Rock Band" On Xbox 360'

While DLC can be a bit more open when you license exports there's a bit more work put into that because licensing on those were originally made for Disc only and later were given an export clause to allow it on future games possibly only within the same console family.

So all that in and of itself is a major headache when crossing over systems.

Then we have to shift gears to talk about delisted DLC. These contracts are fully over and sale for them is not supposed to exist anymore. Harmonix is absoultely tied to whatever agreement they put before hand which luckily seems to allow the moving on to newer consoles however we still get to the issue of only allowing people that have actually already purchased it entitlement to repurchase it for free on a new console.

This requires working very close with Microsoft and Sony to make sure that everyone gets their music properly and the time and work it takes to do this is not free.

Harmonix and or Microsoft already goofed and made some delisted tracks available for purchase at the launch of RB4 and who knows how much that will end up costing them in license infringements.

TL;DR They know what they are doing and are trying to get you your items as quickly as possible.