Incorrect.
The state of Iowa has a law that the casino can pay back no less than a dime on the dollar for non-exotic wagers.... Win, Place and Show.
Every once in a great while you will see a strong horse fit into a weak race in preparation for a larger race. I'm not talking about being a big favorite but very nearly impossible to be beaten to show; barring a DNF.
This encourages large show bets, $100,000+, usually on a Tuesday or Monday afternoon card.
There is not enough money in the show pool to cover this size of action but due to the law, which is usually a nickel on the dollar in most states, the casino is stuck with the tab.
I worked at a horse track to 10 years and remember it happening 3 times, may have been more but I didn't see them.
I believe this is the only instance when pari-mutual wagering is not a lock for the house... and of course this is only because of a legislative act.