Checking certs in a way of insulating themselves from charges should you go and get yourself killed while doing the dive and nothing else. Unless you are in a paid position on the dive or sponsoring it in some way, you have no legal requirement to provide services in your certified role which you are not acting in. Good Samaritan rules insulate you to assist without risk should you choose. I don't know if a certification agency could sanction you after the fact for not aiding... Morally? That is up to you......
If you willy-nilly go off on your own to rescue a panicked diver and get yourself killed it's on you. Good samaritan laws should protect the operator. But, that's wasn't my scenario. I'm talking about an AOW diver who is asked to help and not knowing any better goes to the rescue, gets climbed on, and because the fool didn't have his reg in drowns. Other divers heard this directive so it's backed up in court that the op ordered the rescue and since they didn't check certs, or more importantly at the time ask for cert level, would put the blame on the op. California excepted since it is implied by mathauck0814 there is no cert level that gets signed on a waiver.