If you are a non-elite passenger flying economy on most non-WN airlines, it may require an extra fee to get a preassigned seat, if any are even available (and available together if you are in a group who wants to sit together).
I'm not sure if we're talking about the same thing here.
I fly multiple non-WN carriers, although I prefer the legacy carriers. To sit in economy, you pick economy at check out and select a seat unless you are flying in that airline's version of ultra economy where seat selection is not available. I would not fly any airline's ultra economy version of seating because it matter to me where I sit as well as sitting next to my travel companion. But regular economy seating seems to be easy enough and comes with an assigned seat.