I don't think this situation was a lot to do with weight but more to do with the enviroment they were having to navigate.

It looks like some form of narrow gravel track/ path and the crew would not have been able to guarantee getting the injured woman back to the ambulance for treatment without hitch. If someone had slipped, lost there footing, tripped etc then the risk to the patient would have been magnified and who would have been found at fault if the stretcher had been dropped?