The policy requires airlines check every 24 hours for a new/updated list. This guy was a new addition, so he got missed. And they are now changing the policy to require airlines check for new/updated lists every two hours.
But is it ultimately the airline's responsibility? Isn't that what the TSA (Transportation Security Administration) is for? Every individual person goes through a security screening, and every individual person gets looked at by a TSA agent.
But the government says it's the private airline's fault, not the TSA that they oversee.