I agree it is an authoring restriction. When I order a multidisk title, I often give the 2D BD to my daughter. I'm aware that I lose the extra features but usually, I watch these once and never again, so the disk has no additional value to me.
I do recall once experimenting with my Sony 90ES projector and my Vizio 32" panel and here I can shut off the 3D and watch in 2D if I wanted to. But in each case the player is a 3D player.