Jagex, as we all know, is a multi-million dollar company in a very popular field. One might think that they would make any charitable donations of theirs known to the public, and specifically their user base. But I don't believe that they have done either of those things; I haven't heard of their giving away money, or telling anybody in particular about it. Granted, some donations on their corporate website, but they have never publicly announced these.
Think about it this way: if I play RuneScape, and also have, say, type 1 diabetes (which I do), and Jagex makes a sizable contribution to a diabetes charity, I would be inclined to play more and openly support Jagex more, especially since they supported a cause near and dear to my heart. Even if they supported a different cause, such as autism, I would support them - because although I do not have autism I know others who do, and understand how their lives are made harder by their condition.
Then we have the possibility that Jagex brings in its customers to support a cause. In this case, we have something like a bonus XP weekend with one huge condition: for every, say 1M XP gained by players, Jagex might donate £5-10 to a charity (or group of charities) that was (or were) nominated by players in the weeks before the BXP weekend. This way, players benefit, Jagex benefits, and charities benefit.
The players gain the most obvious benefit: that of a bonus weekend of XP. A large amount of players wouldn't necessarily care about the money being given to charity, but would be glad of a bonus XP weekend, so they would play and benefit others unknowingly. Giving a taste of the weekend to free players as well would give those of us reluctant to shell out US $5 a month a bonus, and maybe tip the scales for some to pay for membership. With the F2P crowd gaining extra XP as well, we would make more money for the charity (or charities).
The public's overall view of Jagex as a corporate entity rises because they just donated however many dollars/pounds to charity to make other people's lives better. Jagex cares about people, and so the people begin to like Jagex more as a company. Perhaps those who like to play games but have never gotten in on it would play to help their favorite charity, and then afterward, decide the game was worth playing on its own.
In the event of a widespread catastrophe, such as the recent earthquakes and tsunamis in Japan, Jagex could certainly make a large contribution to those in need. But it becomes more fun and you become more aware of it when you, as a player, are asked to help contribute by also bettering your own character. This is probably the most obvious application of the phrase 'Everybody Wins.'
Obviously, horrible tragedies are not something to make light of, but when you become involved in a simple way to help others, you begin to think about other ways to help those in need; even if you don't, you have still helped others, and that's what really counts.