Viacom was subsidizing the existence of online in every game. When Warner published Lego Rock Band, they didn't include online. They also didn't include online in the track packs even though it was the RB1 engine where online was "already there", and the online services shutdown disclaimer was even in the credits of the track packs.
Think of it this way: the 80% or so of the RB4 userbase that buys this expansion is subsidizing the online function for the 2 or 3% that will actually play it. Harmonix will probably never say that directly, because it's a bit brutal honesty, but that's probably how they've costed it out and how it's going to get implemented into the game. As a side effect, this also means anyone you meet online is going to at MINIMUM have the New Feature and other expansion content. There are plusses and minuses to this implementation, but weighing all the other ones, this is the least sucky, IMO.