For right now, I suppose I'll be the rule dept. In most PGA and I think all USGA events, golfers are req'd to use the same kind of golf ball throughout the course of a round. In the case of running out of your type of ball, you can run to the pro shop and buy more, or you can withdraw. In this game, you can buy more balls at any point up until the last hit that either uses up or loses the ball, and they will be placed in your inventory and used, even if purchased mid-round. However, WGT forgoes the draconian measure of forcing a withdrawal if one can not make such a purchase before losing their sole ball, allowing players to at least finish the round, albeit with a starter ball.
So what seems unfair on WGT's part is actually a more lenient interpretation of the rules of most professional tournaments.