The main problem is the way of looking at the study, which can provide the main factor for it could get out of control.
For those who have enough finances, they gamble to relieve fatigue as mere entertainment, so they can gauge when to stop and also, basically, they know they won't get rich from gambling. But if you have no finances and consider gambling to seek wealth, then you will be trapped in that, Because gambling will not make anyone rich, because they will definitely experience defeat in time.
Perspective and view point is key, how people view gambling determines their reaction and actions towards it. Just as you've said, since the rich are already rich and do not consider or expect to get rich to get rich via gambling, it's a lot more easier for them to play with a levelled head without placing all their hopes on gambling to render financial assistance to them, so whether they win or lose doesn't really matter, and this is exactly how people should consider gambling.
But of course we already know this isn't the case of gambling with poor gamblers who take all their life problems and place it on the outcome of a bet, so when things fails to go according to their predictions or speculation, they start misbehaving and allow their emotions get the best of them.