Fascinating. What about religious beliefs as a reason? In addition to the ones proposed?
Re: "We are left to wonder why they wanted to help. Did they consider it their duty to a fellow human being; did they feel the pull of friendship--or even family--ties? Or, did they stand to benefit in some way from the unique arrangement? A more thorough treatment of the economic and social networks that connected white and black members of these communities might help us to answer these crucial questions."
Or would that be covered under "duty to a fellow human being?" Family lore has it that one grandfather 1) left his church and helped establish another one when the first one decided to be segregated, 2) stood off a neighboring employer whose maid had fled her abusive employer, and 3) allowed her to live in a small place they had by the house (which had to have been small as well since we are not talking plantation--more yeoman farmer lifestyle. I'm suggesting that social networks were more viable than we might think.