As long as they are not providing private information there's no harm sharing it, some are just happy that's why they are vocal with how the outcome of their gambling, while others have good experienced and manage to share tips but it's always depend from how gambler take their opportunities and edge towards this activity.
Most gamblers losses their money the very reason why we saw a lot of new gambling site that spawning around,
house keeps on taking advantages from those gamblers who don't know how to manage their bankroll the right way.
If the win is huge then it should really be confidential because robbers could attack you but if the win is not that huge then I do not see a big problem with that. Those that love to share their winnings might be gamblers that gambling rarely or gamblers that rarely win.
I experienced this feeling and I am really happy to think that I won after a long time of loosing so I let others know that its possible and they should not give up so that they will also win like me. True, most gamblers lose because of greed, that is why bankroll management is very important even if we only play just to enjoy because gambling is an addictive activity.
Yes greed is one of the major problem that most gamblers do face. Having the urge to win more without thinking about the contrary view. We all need to avoid this or just control it so it will not be a bigger problem later. Share your results and winning and I'm sure no one will attack you or steal your fun especially when they winning is not a large amount.