If we speak about charities, then it should be a free wish, not an obligatory to share only when you have won. Wants to support someone or organization, do it when you feel like you want to do it, not only when you have won a lot.
From other side of the coin, sharing money won in gambling is sort of a motivation for others to try gambling. I think not many would like to be a main hero of a story "how I started gambling". Someone might not understand story about sharing gambling money and would start gambling, because he wants to help people with money.
Actually, I had not thought about it, but you are right. Sharing one's money earned through gambling and letting those people know we got that money from gambling could prompt those same people to become gamblers, because they would have a false impression from the beginning and assume gambling is an activity which is always profitable for the gambler. That could be specially dangerous if we have money away to teens or young adults, which are not psychologically ready yet to face gambling and the consequences of losing money to casinos.
I suppose it would be more appropriate to share that money or products bought with that money without giving many explanations where the manor came from.