From the reports that I have seen, the cops and the military were taking the weapons from houses that were empty, while the houses were being searched for bodies and survivors. That does not mean that they have been frisking the people that remained to relive them of the weapons that they had on their persons.
I find it hard to believe the cops were taking guns from live people, but find it completely believable they were taking them from unoccupied houses. I sure hope they made note of where and who they took them from. So, under whose or what authority did they do this?
I guess you did not see the video of the CHP (yes California Highway Patrol!) officer relieving the little old lady of her revolver. She was just showing the officers, who had come to "Talk" her into leaving, that she had food, water and protection. She was not holding the weapon by the grip, she had the palm of her hand around the cylinder. California's finest tackled her and shoved her against the wall of her kitchen, and then to the ground. The poor woman was shell shocked as they lead her away. She had previously stated to the police "I don't want you in here", yet they remained, absent both probable cause to believe a crime was being committed, and absent a warrant.
Then there is the other video of the police handcuffing, perhaps with plastic cable ties, several people in their fairly upscale (and never flooded) neighborhood. The police took their guns, but did not arrest them, or otherwise accuse them of any crime.
The NRA and SAF have reportedly documented around 40 such cases.
Both the police commissioner and chief of police have been shown on video making statements like "no one but law enforcement may have a gun" and "we are going to take all the guns". Now they both deny issuing any confiscation orders as does the mayor. They made these denials in documents filed in Federal Court, in spite of the video evidence. Federal Judges are in general not amused when people lie to them.