From the article:
"When a waiter adds up the check wrong in my favor, I let him or her know. When a clerk hands me back too much change, I give it back," said George Hobica, founder of AirfareWatchdog. "These fliers knew that this was a mistake, and they should treat an airline the same way they treat any other entity."
I very much agree. Anyone who purchased a ticket probably has a solid legal case that their ticket should be honored or compensation received. But morally, the attitude shown by the people demanding for them to honor it is pretty sickening.
United is also in the wrong for not acting quickly and sending notification to those who purchase stating that it was a mistake fare and asking for them to cancel or rebook.