I think SAG contracts might have something to do with it. I know that they pay full P, which is more than F, so it might be a way for UA to gouge a bit more cash from the SAG's cold dirty hands.

For example, the PS fare LAX-JFK is $4495 one way, while the cheapest F fare (FUAS) is $3665. While there is a "F" fare, I'm guesing that is the full IATA fare which nobody ever purchases.

Still doesn't seem to make a lot of sense though. The only other thing I can think of is historically, the LAX/SFO-JFK flights have always been seperate and might have had this fare designation to differentiate them. Over time, it has not become less meaningful, I suppose.