It is the equivalent of saying "commercials can't be prettier than the program's average prettiness".
Technically, it's the equivalent of saying "commericals can't be of higher brightness than the program's average brightness". Prettiness is a subjective measure, whereas the amount of sound or light energy is objective.
Not that I agree with such regulations. I get commercial TV for "free" so I can't complain and I don't watch much of it anyway. Their noisy adverts have driven me away from most of the programs I might once have watched.
The only influence TV adverts have on me that I'm aware of is in choice of supermarket. I'm definitely a supermarket snob. But when I do watch commercial TV I see no adverts for any of the products I ever buy.