It's not their money to throw away (even to the noblest of causes), it's shareholders' money and should be used to further shareholders' interests.

It would be absolutely unethical and wrong of them to donate more than the normal amount to charitable causes. Now, should those +$100M executives pay more to charity? I think absolutely yes. But Apple the company (or Google, or Microsoft, or any other company) is there to serve it's shareholders not the whole human race.