I was thinking at first that the "mindless AI attack" was implausible, but then I considered scenarios for it to be reasonable.
Consider any system with very strong government control over the military. The government gives the military a mission and expects said units to do the mission, no excuses. Failure simply results in the government bringing a bigger hammer. In a system and a government where the ruling power has long had a monopoly on force, economy with that force isn't as important as ideological commitment to wielding that force in the way and manner directed.
In the case of the "Angels", given a mission to attack a planet belonging to racial abominations, arguing that the presence of a particular technology or ship would require the complete rethinking of tactics and strategy of the empire would be just a tad beyond the pay grade of the commanders of the fleet.
In terms of the AIs values, while the loss of the fleet would be a severe problem, calling into question the judgement of the leaders who ordered the fleet to move would be far worse, in the eyes of the decision makers.
This isn't just an AI politics thing either. If a player empire were given an option that would effectively make it a NON-player empire, it would be very hard for that player to accept, even if the terms were greatly to the advantage of their race and empire.