I can't comment on Australia specifically, Dick, but in general it's a case of a risk-analysis done by the aircrew and ground teams on-scene.
It may be that the casualty has only minor injuries (And I don't agree that a fractured leg necessarily constitutes a minor injury when you're outdoors) but a minor injury can very quickly become a serious issue when you are outdoors and exposed to extremes of the environment.
One also has to consider the extended risks involved. You suggest evacuating them with ground teams on a stretcher. All well and good but that implies that you have sufficient people to do that and time. It also places a different kind of risk onto the rescuers - as anyone who has ever carried a stretcher over even slightly rough terrain will tell you.
Time is also a factor - even if it does not directly affect the severity of the casualty's injury, the more time that the team have to spend dealing with him is more time that they have to spend in a potentially hazardous environment and less time that they are available for another incident. Sometimes just getting them out of there with the helicopter is the best thing to do.
At the same time you have to be sensible. As an example I'm reminded of a case a few years ago where a UKCG SAR crew decided not to winch a fisherman with a comparatively minor hand injury because they felt that the helicopter would have been very exposed in the event of a failure of any kind. They asked the vessel to steam towards land and went back later with less fuel and a greater power margin.
Yes there is a risk to everything but I'm not aware of a massive number of engine failure related accidents in the SAR community and as we see the introduction of newer machines with better single engine performance then this should become even less of an issue.
OH