Immoral or not, it depends on how we see it. There are laws prohibiting unfair or deceptive practices in advertisement, but they are different in different countries. And is showing your real wins on slots unfair or deceptive? I don't think so. As with any other advertisement, it is we who can make it harmful for us, not the ad itself. Ads are annoying sometimes, and in other times they make us buy unnecessary things, but the world would be less funny without them. And these big wins on slots, in particular, some of them are really fun to watch. 
That simply talks about money and even it was immoral, they let it happen as it was paid already. It is sometimes to think that was too unfair, yes, it was but have less possibility that we can change the nature of this place. Then rather we saying it was too unfair, too immoral, maybe it was best to ignore them than be a big deal to it. It is actually annoying when we heard such complaints but can't just put blame on them(ads) alone, we do it for ourselves then for listening and choosing them.
It always our fault and not anyone's else that we are so greedy that we are ready to believe that we are necessarily going to win like those guys in the vids. We don't want to think about how much money they were risking, how many times they were losing before that big win etc., we just want to win like them right away. It is no wonder we are disappointed and broke, more often than not, following those expectations.