There are many states where trucks only need one plate, and they put it on the front.
Now the supposed reason is that you might have a trailer attached sometimes. But since most truck drivers rarely ever haul with a trailer, the reason is pointless.
In my experience, white dudes follow this "rule" here and people of color put the plates on the back, you know because cops are racist pigs.
I for one am tired of tarnishing the good name of pigs by associating them with cops.
Pigs give us ham, and pork, and bacon. Pigs are at the center of every good barbeque. Pigs eat garbage and produce wonderful cuts of meat. Pigs are wonderful, smart creatures and don't deserve to be cast down by permanent association with the worst and dimmest.
Cops are locusts. Inedible, swarming creatures that descend upon decent folk and blight all they touch.
I know in some places (EU) and maybe even some American states that law does exist, however judging by the truck and the cop car this image was taken in the south. In my home state of Georgia the only requirements to register a vehicle are two headlights, two taillights and turn signals on the front, back and rear. That's literally it. I have actually seen boats driving down the road sitting on top of a truck chassis. In fact those rules get even more lax if the vehicle only has three or less wheels in which it only needs 1 light and 1 brake with turn signals in the front and back.
Their lights are on, the pigs probably rolled up behind him at a red light, took a couple minutes to finish reading the line and then decided to pull him over.