I am not disagreeing with the comment directly, however I had an N6 shipped to me immediately after release (actually 4 of them), bought from Motorola direct. Opened the box, pulled out the sim tray, put my Verizon sim in, pushed it back in and it was fully active on Verizon that very same minute. I know they weren't "officially" supporting it, but they weren't preventing it either, so it's not really accurate to say they weren't "allow[ing]" them since they did allow those Google and Motorola versions with all radios installed onto their network, or perhaps better said, didn't "prevent" them from access to same.
No, they weren't offering one for purchase a the Phone Center Stores, but that's as close to not allowing them that they came.