I'm not sure if voiding all the other bets is the best policy to take here.

They seem to be in a real rush to close this entire operation down which makes me wonder if they have run into trouble with some sort of authority/regulator.

I think that the more transparent thing to do is probably let all of the bets settle, and simply not allow any new bets to be placed. Instead of voiding without warning all the bets that have been placed (potentially years ago on long shot events).