There is plenty of good reasoning executed badly from a PR perspective.

It is (now) called the Department of War for a reason: It needs to be able to kill people very fast and cheap. Autonomous weapons platforms do that: you give them a geofenced area and let them kill everything that moves in said area. No loss of human pilots, no fatigue.

If Anthropic they had any ethics concerns they would not have signed up the Pentagon as client in the first place.

My guess is that they wanted to have their cake and eat it too.