Why? Facts are facts. The definition of a fact is that is an objective, verifiable statement. Trust does not come into play because you don't need to trust anyone judgment to verify a fact. Judgment is not involved.

Whether Fox fired Megyn Kelly or not is a fact. Facebook's is judgment is not involved in verifying it.

Whether an FBI agent that examined Clinton's emails is now dead or not is a fact. Facebook's judgment is not involved in verifying this fact.

You are conflating fact checking with opinion editing. The suggestion here is not that FB should filter out any kind of objectively verifiable fact, no matter which way the fact is used. It's that perhaps FB should filter out or at least not signal boost objectively verifiably false information.