Why is there a “2D” disc in every Blu-Ray 3D package?
Disk space.  The 3d content takes more space so there is generally not enough room for special features.  The 2d special features are only on the 2d disk.  They take up the room that the second eye takes on the 3d disk.  They could make a special features only disk, but this is generally more expensive since it requires another master and another production line.  It's more cost effective and better marketing to just include the 2D copy.