George Weinberg:

Does it occur to anyone else that the fable is not a warning against doing favors in general but of siding with "outsiders" against "insiders"?

Wow; now that you mention it, that is a blatant recurring theme in the story. I now can't help but think that that is a major part, if not the whole, of the message. Each victim betrays an in-group to perform a kindness for a stranger. It's pretty easy to see why storytellers would want to remind listeners that their first duty is to the tribe. Whatever pity they might feel for a stranger, they must never let that pity lead them to betray the interests of their tribe.

Can't believe I missed that :).