When a team doesn't field a team, the opponent is given a 25-0 WO, but then some lucky fans come down a play a game, which can end up with outlandish scores like 362-13. The fake game is played so the other team gets training minutes and also gets some stats for figure league scoring leaders.
But this can skew the stats, since an ordinary player might put up 50 or 60 points against the lucky fans. And the 25-0 scoreline might have an effect on league standings, since tiebreakers include points scored, which a 25-point game won't help, and even the 25 point point differential might be less than a good team can put up against a weak team, or could give a advantage to a team that ordinarily has close games.
So why are there walkovers? IRL any commissioner is going to step in if a team failed to field a team, especially when you have several 1000 fans show up. If there are strategic advantages to giving up a walkover, there shouldn't be. Otherwise walkovers are generally due to carelessness, such as forgetting to set a lineup, and not having a valid standard lineup after some transfers were made. In these cases, there is no reason to have a walkover if it can easily be avoided.
So how to prevent walkovers:
(1) Minimum roster size. If a player firing would leave the team with fewer than 10 healthy players, or if the transfer of a player would leave a team with fewer than 10 healthy players, the transaction will be blocked. It is reasonable for the commissioner to step in to maintain the integrity of the competition.
While there can be some inconveniences, they are generally minor and can be worked around. At worst there might be the option to bid up to $10,000 regardless of debt in order that some lugs can be hired, so that a team will be able to sell a more valuable player in order to generate a positive balance.
(2) Ensure valid lineup.
(a) If the lineup set for a game doesn't have enough healthy bodies (8?); use the standard lineup.
(b) If the standard lineup doesn't have enough healthy bodies (8?), choose up to 12 players at random from the roster.
(c) If there still aren't enough healthy bodies (8?), hire some walk-ons. These will be similar to Lucky Fans, but they will have an actual identity, and minimal skills along with minimal salary.
(d) If there are injuries, foul problems during a game, lucky fans can still come down and play.