I do think your hypothesis is plausible, but the reasoning it describes seems too complex. One would think like that only if one cares about being consistent and reflects on that, and only after one has decided that the "I've done my part of the job" excuse is not enough... and it seems improbable that most people think like that.

Also, it seems to me that "help Haiti just this once" is not the same scenario as "help just this person".

Worth testing, though. I guess if you set up a scenario like "Help poor kid X grow up well", a long term goal with kinda-hard-to-predict cost that most people wouldn't be willing to pay all at once, with a specific identifiable subject...