They teach metal bending to police earth benders whom have earned they're way into there or those who have connections (nobles and or long lasting earth bending families) along with some skill.

 Earth bending in this roleplay will be balanced out to make it fair for other purist earth benders by having purists be overall better at controlling Earth and can use it more versatiley while metal benders have their name sake and some rather decent earth bending to fall back on. An example of this would be this. Imagine an Dai Li (Earth Bender) or Earth Bending Elite Soldier vs. A Metal Bending Cop Captain. It's a question of being better in one area or have multiple areas to have at your disposal.