Can't see what possible grounds they have to refuse paying.
i can see 7.3 million reasons.
Having worked briefly as a croupier I would say casino staff have about as much understanding of the odds / probability / luck as the average punter*. Hence I suspect their default position is - How can he have won that much? Thats impossible he must be cheating!
*One pit boss was consistently telling me to change the speed I was spinning the ball "so it hits a different number"