The same thing that gives any person the right to do whatever they want with their money. Contrary to popular belief, companies don't have the obligation to restrict themselves to a particular business nor do profitable things. They can donate the amount they want to the charities they want. If, as a shareholder, you don't like it, vote the board of directors out.

> Also I'd imagine the only reason any major company donates to charity is to strengthen its brand in some way.

Not everybody who bites their nails does so because they are sexually repressed. Not every bourgeois makes organized and self-conscious efforts to push down the workers' revolution. Not all people feign compassion and altruism, but are just jealous of the strong-willed nobles.