Because this isn't just a private matter. Peter is very vocal about his vision of society - in which tech plays a big place - and this donation definitely gives some interesting color to that vision.
My grand parents lived through the second world war and food shortage was a big issue during and after the war. That's why my parents and grand parents always taught my siblings and I to respect food and avoid any waste.
I believe that's very common among people my age (40) in Europe. I wonder if that's just less true of Americans and younger Europeans because hard times are further away from them.