Bear in mind, the "morality points" aren't on Google: They're on Google's employees. The commitment Google made for AI Principles only exists because their employees demanded it, and forced Google to end it's participation in the Pentagon's drone warfare project.

Google's management are more than happy to help the government engage in illegal assassination abroad via drone, but it's employees are not. For Google's executives, this is less to gain morality points with the public, but to retain employees who would leave otherwise.

Though another aspect is, they may have already gotten an indication from the DoD that dropping out of Maven would be held against them when evaluating the proposals. RFPs aren't just about the dollar value, and they'd certainly take into account the possibility of Googlers refusing to do the work needed to fulfill the contract.