It is illegal to use those number plates, you can hear him say that she has a citation for those plates from yesterday, which means she had been ordered to remove them and use the actual ones and still refuses to. Since the plates are illegal, the police are legally allowed to confiscate them. He is heard asking her where the actual number plates for the car are.
Still, He should not have torn them off like that, if the car got any damage in the process he would be at fault. Otherwise he was highly professional, good thing the body camera is there.
I assume he took them off angrily because of her rude behavior already and from yesterday, and because she refused to stop when they tried to stop her (they had to chase her down, you can see the police vehicle on the left had to box her in to stop her.)