I don't play horses, so i am not at all familar with how other books handle their payouts. But there seems to be an inherent unfairness here.
Had the player lost, WSEX would have kept his full stake amount. But when he won, they only paid out about 5% of the winnings. (you'll recall there was a similar situation with intertops, when a play bet a future on the marlins to winn the world series, but the win amount exceeded the max pyaout amount) If WSEX were to readust all the wagers, win or lose, so that the final win amounts are at or below there cap, then I would say it's fair. But in this case they took a wager that they were in no way prepared to pay. The software should have rejected the wager.
Maybe horse wagers ar eused to being screwed. But in other sports the sentiment tends to be, if the book was prepared to keep my full stake if I lost, they have to be prepared to pay in full. Lastly, in fainress to the player, it is not at all uncommon to see promos where the rules of the promotion supercede the general rules. Just as an example, Nojuice has a rule of a max $500 bonus per 30 days. for Breeders cup they are offering a $1000 max Bonus Play bonus. Now in theory it should not be possible to get the $1000 bonus since the rules clealry state a max of $500 per month, but the promotion is the exception to the rule. It's not unreasonable for a player to assume, that if the book is advertising full track odds, that it means that the caps are going to be lifted.