Given that the US isn't a perfect capitalist system (i.e. what supports of capitalism often refer to as corporatocracy or crony capitalism) why would you assume

Given that the US isn't a perfect capitalist system (i.e. what supports of capitalism often refer to as corporatocracy or crony capitalism) why would you assume that he didn't do the closest thing to illegal that he could get away with if it was most profitable to do so? That said, the actual reason for his philanthropy seems to be fairly well documented: he did almost all of his philanthropy in the last two decades of his life (i.e. past what we would now call the age of retirement) and he was regarded as a shrewd business man up to that point and is now instead remembered as a generous philanthropist instead. So in other words, it was a very successful PR campaign to establish his personal legacy to survive his physical existence (i.e. "to secure his legacy" as the comment you originally replied to stated). That said, he was also supportive of the ideas of progressive taxation and an estate tax, so the philanthropy may have simply been an act of last resort to part with his money when the government won't take it away through taxation. In that light it sounds more like his philanthropy aimed to meet the needs the welfare system failed to address. This is very different from the kind of philanthropy most billionaires (or "tycoons") get/got up to who are often very much against being taxed more (sometimes even despite publicly stating the opposite).