Opportunity for UA
In these type of situations where E- is completely full and it is likely that the passenger is going to end up in E+ who paid full fare on a refundable ticket, an offer of a complimentary upgrade to E+ by a check in agent would make good business sense. Chances are they are not giving away anything that will cost UA money and there is a chance that the free upgrade to E+ given by someone who thanks the passenger for their business and makes it clear they are getting a free upgrade to a better seat, could result in future business. To do this the check in agents would need to be empowered to do this and care about presenting a positive impression.