Aviation safety for FAA airworthiness generally follows SAE ARP4761, which requires risks of Catastrophic severity to have a quantitative failure rate of 10^-9 incidents per hour or less. The Air Force follows MIL-STD 882E, which generally requires risks of Catastrophic severity to have a quantitative failure rate of 10^-6 incidents per hour or less. In short, the military accepts more risk than commercial aviation. That being said, military aircraft usually are also rated to ARP4761 levels of safety so they can fly in US airspace.