Well, we're talking about working on software projects here. I work in defense and would have no qualms about the whole Project Maven thing. But if all they want is the freedom to decline to work on it, that seems pretty reasonable to me. They didn't sign up for that stuff, and Google isn't primarily a defense company.
The draft is (in theory) an emergency measure for the good of the nation. Just like the government can force you to pay taxes, they can force you to fight in the military. That's certainly not a power a private corporation should have.