AA absolutely does this. The longer I choose to sit around DFW, the cheaper the fare, regardless of when I leave (early or later) or the flights I'm on. If it's a double connection, it gets even cheaper - then you just have to decide which of the two airports you want to sit around for 4 or 5 hours. I have no idea why they do this.

What UA is doing is a bit different where they are offering combinations that normally wouldn't come up as a sensible option. I saw this the other day with fares laying over in Denver that were just end on end combinations of very cheap CLE-DEN and then DEN-PHX (for example) flights, but the end on end rules seemed to prohibit a connection time of less than 4 hours. (You could purchase the two trips on two different tickets/records and have a shorter connection.)