Maybe there should be a law (let's call it the Dao/Amrich Act) where you cannot remove a passenger from a flight once their valid boarding pass was successfully scanned at the gate, other than for criminal reasons (physical assault, smoking, verbally abusing the crew or other pax, etc -- that pushy family in another thread insisting others move for their benefit, would qualify for ejection in my opinion). That way the onus is on the airline to stop a passenger before that point by not issuing a boarding pass at the check-in process, not scanning the boarding pass in order to deny entry at that point (the wiseguy in Iceland...), or the boarding pass comes up invalid at the gate and the pax is refused entry that way.
Then neither of these incidents would have happened. Under such a law, Dao would have had to be bumped from the flight at the gate, and Amrich would not have been able to get a boarding pass (giving her enough time to maybe straighten it out with the landlord?)
As much as we all want to pour on the hate on United, what could the gate agent do? She was ordered to remove a pax whose ticket had been cancelled. That's all she had to go on. She needs to be reprimanded for the "nobody flies for free" comment, but I'm sure she's met every wiseguy/fare cheat/RFD'er/scammer and heard every sob story out there so it's hard to blame her for treating this one any differently, even though this was one time she would be wrong.