If they can check the machine, they should have done so before allowing people to play. After all, they aren't paying back the losses on the "malfunctioning" machine.
It's to make sure the player or other third party hasn't messed with the code. And if the machine is paying out less than it should, there are penalties from the gaming commission.
If this was to prevent fraud, that would be what the signs on the machines say. "Malfunction voids all pays" is to minimize payouts, even when the gambler is not at fault, e.g. buggy software, non-verified software, unrelated tampering or tampering attempts. It's sufficient that "something" is wrong when the jackpot is won.
The argument I was trying to make was an ethical one: By accepting the gambler's money, the casino should take responsibility for the state of the machine at that point, thus incentivizing them to be diligent. To void a jackpot, they should have to prove fraud by the gambler or their associates to a court of law.
