Apparently the publishers have determined free giveaways boost sales. Or maybe just get the message out and evangelical considerations trump monetary returns. Who knows?
In any case, I'd say that the erotica people have determined the same, given how many freebies they give away as well.
This one is non-free for Canadian Kindlers, by the way.
ETA: Might as well add that it's also free in ADE-DRM ePub over @
Christian Book (not available to Canadians).