Welcome to FT.

One answer is that there's a lot less competition to TYS than there is to LAX or SFO, so UA may be able to make a profit on lighter loads with higher fares. Note also that seating charts are notoriously unreliable for measuring availability. And trust me, the rules about Saturday night stays, round trip fares, 14-day purchase, and so on are very much alive, especially in smaller markets.