Sony tests for a lot of things when they test DLC, and their testing pipeline is longer (a good example is the Who Superbowl Smashup delay), so the limit helps keep that somewhat reasonable and doesn't piss off Sony.
Amberian Dawn sells, so I can understand that. Same with JoCo and Bang Camaro. Even the Main Drag I can understand. But other bands have had some pretty vocal demands too (Gomez, Apples in Stereo, and Free Spirit are 3 I can think of off the top of my head, I think Rose of Jericho had some followers). We didn't get any or all of their tracks, and they had far less than the Main Drag in RBN. Apparently we are even missing 1 Main Drag song that prevents a goal from being finished.
Now, I'm glad we're getting any RBN, even if I'm not heavily invested in it (I think I only have 24 RBN songs, including 2 RBN2 tracks), but it would be nice to get a bit more or a bit more variety, and perhaps a few more forum favorites ported over like they mentioned they might.