Monsters Appear in Tall Grass

Lands that observe this Law spawn monsters at random from a list that each piece of terrain dictates. If a Block of Tall Grass is acquired from Pokemon near Pallet Town, then pidgeys are likely. If a Block of Tall Grass is acquired from an endgame mountain, it might be a graveler. If a Block of Tall Grass is acquired from Skyrim, it might be a sabertooth.
Law originates in Pokemon. Were you actually surprised? Other Laws have to do with monsters spawning in other ways.