There's one other possibility nobody else has mentioned yet - what if nobody else was around to even hear the scream? What if someone heard it, but nobody knew they heard it and they couldn't be called as a witness?
Rapists don't only use threats to stop the victim screaming, but they tend to choose areas where there's nobody around to hear.
In any case, that biblical rule is obsolete - it was formulated in a time when cities were vastly smaller than they are now, when there was little ambient noise to drown things out, and when communities were very much more close-knit - in other words, it is far, far less likely that screaming for help today would either be heard over the everyday din of mechanised society, or would attract anyone inclined to help - an unfortunate side-effect of the increasing densities and mobilities of populations today is an increased estrangement from the overall surrounding society, leaving less moral imperative for community support.
What many nostalgists tend to view as a noble altruism in ancient, pre-industrial societies ("People didn't even have to lock their doors in the good old days", etc) is more likely an unconscious manifestation of support for one's own social group, simply because in smaller, static settlements with simpler, mutually dependent economies, any random person needing help is very much more likely to be known personally to you. Today, the reverse is true; any randomly selected person about you in public is overwhelmingly likely to be a total stranger and, unfortunately, even modern human beings, who might even support altruism in principle, still have a very strong evolved instinct to be wary and suspicious of strangers. Ironically, this mistrust of unknown persons is self-validating, since one is probably justified in being wary and suspicious of another human being as long as it is a guarantee that they also have this inbuilt reaction to encountering you. It's a reverse chicken-and-egg problem: which can we get rid of first, the cause or the effect that necessitates the cause?
I can't remember where I was going with this, so I'll sum up: 1. Humanity still has a long way to go before we can really claim to be better than instinctive animals in a great many respects, and 2. The biblical law quoted in the original text is stupid and obsolete when one attempts to apply it to modern society. It is a perfect demonstration that dogmatic regression to more primitive social systems and laws, from some perceived golden age, is not a useful way to respond to the new problems arising from the progress of human society, even if they do resemble problems that the antiquated laws were formerly able to deal with.
