Look at the effect, not the justification: just when OO XML was being talked about being standardised, and .doc was more or less interoperable, Open XML comes o

Look at the effect, not the justification: just when OO XML was being talked about being standardised, and .doc was more or less interoperable, Open XML comes out. It's an open standard, but it requires effort to convert to it. Microsoft has money to burn so it can do the conversion; its competitors cannot. Copyrighting the file format puts them in legal hot water: since it is a de facto standard, trying to prevent competition would likely violate anti-trust laws. This way they can appear to be open whilst being closed.