Here's my 2c. In my opinion (which is not a fact), UA's action was fair enough.
When you make a mistake which negatively impacts someone (e.g., makes someone lose money), then you need to pay some penalty for the result of the mistake.
However, this mistake by UA is different. Everyone with a normal brain definitely knows that 4 miles for a round trip is a mistake (again, my opinion, not a fact). People just want to take advantage of UA's mistake.
There's nothing wrong with taking advantage of others' mistake, but meanwhile there's no reason to get mad when they discover the mistake and try to stop you from taking advantage of it.