In certain states anything that blocks or covers any part of the plate is illegal.
Even a clear plate cover is illegal here.
Call it what you will, you weren't there. So you are only getting one side of the story.
Here is the USA driving is a privilege not a Constitutional Right.
So it can be regulated beyond belief.
That's the reason the Penal Laws are around 500 pages and the Vehicle Traffic Laws are about 2000 pages
HTA Section 13.2 ”every number plate shall be kept free from dirt and obstruction and shall be affixed so that the entire number plate, including the numbers, is plainly visible at all times, and the view of the number plate shall not be obscured or obstructed by spare tires, bumper bars, any part of the vehicle, any attachments to the vehicle or the load carried.”
it's ambiguous cause the main focus is on the numbers need to be visible. Even without a frame the heads of the screws or bolts that you use to secure the plate cover part of the plate so I guess the cops could ticket all of us. Like you said we are only getting one side of the story but are you suggesting that the cop was unable to give a ticket for something else he/she chose the plate?