I'm going to take a wild guess on this one. People who drink moderately are probably more extroverted than people who don't drink at all. Extroverts are by their nature more involved with others. It wouldn't be surprising to find that they would be more inclined to want to help those that they perceive in need.
I am really looking forward to reading this book.
Interesting. My own theory was that the drinkers were more likely to be Catholic, and we pretty much own the guilt department, which leads to giving a little more money.