Trying to PX four crew members last minute isn't considered unforeseeable when it relates to Force Majeur, to suggest that in court would be laughable.
Unforseeable would be something like a meteor leaving a big crater on the runway and indefinitely suspending operations.
They can't possibly list every single act of god on the contract and hence the wording of " but not limited to" or "unforeseeable". That clause isn't written to grant United operational flexibility by allowing them to remove passengers from a flight.
Of course any contract is open to interpretation, hence why we have lawyers, judges and courtrooms. But the majority of legal experts are fairly certain that United in this instance is liable and at fault.
You seem to think that because an airline has a carriage of contract that they can do whatever they want.. it's simply not true as they are about to find out, but my guess is that this will be settled out of court with a hefty settlement sum.