It has nothing to do with pacifism. It has everything to do with their E for Everyone rating, which has certain rules to obtain it.
I agree, however, unless you know to ask that particular question, it’s not obvious they can’t sell aircraft with weapons on them, but that you can purchase aircraft with weapons directly from developers.
But then again, if Microsoft wrote down all the disclaimers and information that’s important to know, the document would be just as often read as any given set of rules and regulations document you agree to all over the internet.