I will make a caveat that I think that all government employees should NOT put ANY type of little saying on their professional & official e-mails. However, that said, until there is such a total ban, the prosecuton of this corporal should be done, because it is religious persecution by the USMC.
Having a long career in the military, I disagree. It is very positive for morale and discipline to permit individuality as much as possible. One of the great historical advantages of the US military has been the independent thinking of its troops. In contrast the soldiers of other nations are brow beat into lockstep, and that actually makes them predictable and afraid to find solutions in the middle of difficulty. They always try to get approval before they do anything toward achieving the objective on their own.