I think the pre-payment plants a seed of 'guilt', even if unconscious, that will elicit more replies. There are probably studies that back up the effectiveness of even a token implicit "contract". It's not based on logic or the actual worth of what is offered.
Same deal with those charities that send unsolicited stickers or greeting cards. They might make you feel like you "got something" for which you then have to "compensate" them for, to some extent, even if the items are worth zero or close to it. I'd imagine people to have grown more cynical over the years.. but the fact the organizations still send these things out means it's still working. Direct marketing is very very scientific these days.