I agree the OP is entitled to a refund if he/she wanted it - and not taken the flight. Or the OP is entitled to a refund of the difference between the fare paid and the lowest available economy fare on that route now that he/she has taken the flight. But since that flight was under the discounted economy column, that may have been the cheapest fare at that time. That's it for the "entitled to".

As for the "taking care of the customer because this was really because of a UA bug" it would be nice if the company treated the customer above and beyond this basic entitlement. I'm not holding my breath any, though.