Stranger rape is already supposed to be pretty rare. Maybe I'm attributing too much rationality to them, but I figure anyone who puts that much effort into a plan to commit stranger rape probably intends to make money off of it beyond the basic mugging gain of "whatever the victim had on them". Bear in mind, though, that they might not be competent enough for their plan to succeed - but if the victim's endpoint is "dead in a ditch", I assume that comes down to the criminals' incompetence.