It is an exploit, it clearly isn't intended for players to farm these zones using resets to quickly go in and loot and leave.  It's in the same vein as using small terrain bugs to not allow mobs to hit you due to pathing.  It's "clever use of game mechanics" but never intended to be used this way.  Chests have a "long" respawn and resetting was bypassing this, that's where the exploit really comes in.  Resetting and killing the mobs isn't the exploit it's the chest.

That said, I did it also and it needed to be removed.