Is this any different on WN that it is on AA, UA, DL, US, etc?

Technically, based on the contract of carriage, it appears what one purchases is a confirmed reservation. The purchased reservation becomes a confirmed seat once you comply with the airlines check in requirements and are given a boarding pass. After you have your boarding pass, you can voluntarily give up your seat for compensation if the flight is oversold, or you could be involuntarily denied boarding in accordance with Federal rules.

From SWA’s contract of carriage: "Confirmation of Reservations. A reservation on a given flight is confirmed by the issuance of a Ticket." [I make the assumption that the ticketless e-mail confirmation which includes a ticket number is the equivalent of a “ticket”.]

From United’s contract of carriage: “United flights may be overbooked, and there is a slight chance that a seat will not be available on a flight for which a person has a confirmed reservation.”

From AA’s Contract of Carriage: “If a flight is oversold (more passengers hold confirmed reservations than there are seats available)….”

From Delta Airlines Contract of Carriage: “Once a passenger obtains a ticket that reflects confirmed space for a specific flight and date from Delta or an authorized Delta agent, the reservation is confirmed even if the record is missing from Delta’s electronic reservations system, unless Delta’s records indicate that the reservation was cancelled after the ticket was issued.