The entire point of these fares is not to sell them. They're teaser fares, designed to show up in the search engine and then UA hopes to be able to up-sell the customer from there. They likely consider a customer booking AA a win, as it saves a seat for someone who will buy-up.

I'm not necessarily defending this policy -- it seems short-term focused. I'm just explaining it. UA feels there are enough customers to go around, particularly at fares they consider to be low.