There is a WN policy on "saving" seats. It's called "open seating." It a seat is empty and not assigned, it's empty and available. Since WN doesn't assign seats, if there's no person sitting in the seat it's empty and it's available.

True, it's lousy sitting next to a jerk, so most people just move on, but the policy seems clear.

Failure of FA's to enforce is yet another reason never to fly Kettle Air unless it's the only & last thing available.