Why is it that Lethality on cunning doesn't have to scale per level?

Because Lethality is Arm Pen per Enemy Level.

Lethality already inherently has a portion of it scale per level as is.

Lethality = arm pen scaling with level old mastery gave arm pen scaling with level new mastery gives lethality

Well, actually, lethality scales with opponent's level, but still seem pretty fair to me.