Same reason ebay do it - to ensure a clear indication of where the image is against what is generally going to be a white background. Without it - the image can possibly look lost. It could also be the case that ebay only want to generate one image to save on space - and given google requires one that has a grey overlay - they may as well use the same one.

Not to say it can't be a white background - amazon have white backgrounds on product listing pages (though they do apply a grey filter on list views).