United sees a cancelation as a way to keep the customers money and squeeze them onto another flight that had an open seat. Thus increasing load factor and profit without spending the fuel and costs of actually operating the original flight.
United does not (yet) see that the customer that they screwed over, The customer that they told was "lucky UA got them a seat at all", that customer isn't going to return to UA for any flights any time soon.
The entire culture of this airline at the present time is very, very short sighted