Legally speaking, airlines, and their employees, must act with a high degree of care and use the vigilance of a very cautious person in order to protect passengers from potential harm. They must have policies in place to adequately protect passengers--that's another reason for seatbelts on airplanes. Each airline makes its own risk assessment for the risks its willing to bear and sets its own policies accordingly. Mainline US airlines seem to have settled on strikingly similar standards; carriers in other countries set their standards based on their own assessments and different research assumptions.