Could be that the it took about 24 hours for United to decide what should be done about the "mistake fare" and Sunday, Corporate offices are not open.
Therefore, all those who flew on Sunday lucked out. I don't think it is called discrimination against those who got to fly for 4 miles yesterday and the rest don't today or later. This error wasn't talked about till the next business day which is Monday.
Just my 2 cents.