He didn't say they just checked the idiots. He said "DFG and ATF agents were dong a firearms check on everyones guns..."
So, there you are at the range. Joe Good-citizen. Cop comes up and wants to see your ID, and check your guns to see they are not illegal and are not stolen, and you don't have a problem with that? It was okay to violate the gangster's rights because they were criminals. How about everyone else at the range, whose rights were also violated? That's okay?
Once again you are on public land. If you are walking on the street and a cop pulls up and wants to see you ID to find out who you are, you have to show it to him. He has the full right to run your background. If you are a Felon, you have no rights to begin with. If your not a felon and your not wanted, its a simple goodbye. If you are on private property that is when they need a warrent.