Where in the COC does it say that it cannot be used for that reason? The COC cites force majeure/unforeseeable conditions as a reason to refuse transport to any passenger at any point. Can't find anything like "except for ___" to limit that clause.
Perhaps it would be easier to understand your point of view if you could outline how the 'force majeure/unforeseeable conditions' exceptions help you personally? What is the benefit to you (or me) in allowing UA to invoke this exception?
In any event, I would disagree that a sudden crew movement is unforeseeable. They happen daily across the airline network and are part of running the business. Inbound flights are delayed daily, again, part of normal business operations, and crews get accommodated accordingly. It's worth noting that these exceptions were not mentioned or relied upon by UA at any time in their justification of the removal of the passenger.