John Arnold and his wife, Laura, are two of the most interesting philanthropists in America. | Courtesy of the Arnold Foundation John Arnold does what so few of the wealthy are willing to do: Criticize a system that benefits them. One of the country’s most intriguing billionaire philanthropists is sounding the alarm about a highly controversial tax loophole that has advantaged his class of donors over the last decade. Donor-advised funds are the philanthropic middlemen that house over $100 billion in assets nominally slated for charity — well, eventually.