Whats the difference between creatures and enemies?
Creature is anything you can possibly attack.
Creatures = You, your allies and your enemies
Enemies = anyone other than you and your allies
This.
Note that "enemies" technically includes neutral parties.
The additional nitpick would be enemies must also be legitimate threats. A sack of rats is not an enemy.
I dont think the power has the sentience to actively determine things for your players, rather it becomes a concious choice of the player who is or is not considered an ally for the purpose of area of effects.Incorrect. As an example: If your cleric uses a 'target all enemies' power in a room filled with hostages that are tied up, and non-combatant, and do not know nor have reason to trust the players, then that power will treat them as enemies. Hostages are now dead. Your Cleric is now sad.
The Bag-of-Rats rule does nothing towards the ally/enemy relationship, all it does is disqualify using non-threats as 'lightning rods' for the secondary effects of powers that may benefit the party. That doesn't change the rules on what an enemy is.