They could investigate and determine that they did nothing wrong, but they aren't the final arbiter of justice. This is not like the police investigating themselves. The OSS community can come to a different conclusion and still punish them.

The job of UMN is to get the facts first, and then determine actions. If the OSS community (and others) feel like the actions don't match the facts (or if they feel the facts don't match what actually happened) they can apply their own set of actions.