Had to do with shipping. If I remember they can't ship it back again after a reticle swap for whatever reason. Could be off, but I remember it had to do with something like that.
The MR4 is the most desired for resale. So you will make a few bucks selling that and buying a MR5 or MR2 reticle. If you can find someone selling the others. Don't see them popup too often.