I haven't read the thread, just the OP. I don't think they or anyone else "owe" anything to anyone. They both employ plenty of people and pay a ridiculous tax rate so even without giving a cent they are helping with their taxes, just like all of us are.
Of course if they choose to give I think it is great. Anytime someone with the means goes above and beyond and contributes to charity or even sets one up it is great and I applaud them. I do feel though that it is and should be a choice, not an expectation from anyone.
I also don't think they need to give to everything. There are some charities I donate to and some I don't. No one can give to everything and there is no reason to think that people who do give to one cause don't care about the causes they don't give to. Any of us who donate have our own reasons for donating to the charities we choose. There are also a lot of people who give anonymously. For all we know wealthy celebrities donate all the time to causes but we don't know because they don't feel the need to stand up and broadcast that they do.
I also don't think what one gives is dependent on what another gives. What I choose to donate to a charity is not dependent on what my friends or a celebrity donates and vice versa. This is also true of an organization or a country. Just because we give doesn't mean we should expect anyone else to follow suit.
Bottom line, charity is great, needed, and if you can give you probably should. It should always be a choice though and not a mandate by any of us.