Even if we accept the premise that he doesn't deserve any credit for his donations because it is his duty to society, I still find that it is more useful to admire and encourage this behavior from a practical standpoint. At the end of the day, after paying his taxes and nobody can legally lay claim to any of the money and wealth he possesses, he has no obligation to give any of that to anyone else or use it for any purpose whatsoever. So if admiring and applauding billionaires for donating even small portions of their wealth encourages billionaires to do so, that's a cost I'm sure society is willing to bear.

This completely sidesteps the fact that not donating all of his wealth immediately allows him to potentially grow his wealth in other ways while still donating an objectively large number of resources.