Do you have a source on that? I have been following Android news related to the govt and haven't seen that. I am aware of SEAndroid from NSA but not about devices running being allowed on DoD networks.
It's entirely possible to run Android on DoD networks, it just takes a lot of work to certify it and the restrictions of what you can do with the device make it almost useless.
No official source. Just conversations with friends who work for various agencies when they abruptly switched despite professing their undying love for their iPhones.