If you're a defender of a siege and you 'retreat' you are actually surrendering.
But if an attacking army has your castle sieged, and they decide the battle's not going as they intended and walk away, they aren't losing anything. It's not like the defenders came out and captured the spawn of the attackers...you don't all walk up to the castle walls and throw the peace flag up (That would be surrendering, not retreating).
If an attacker in a siege retreats, they shouldn't be considered "surrendering"...one of the dumbest things I've seen in strategus so far.
I could see if it's an open field battle and one side "retreats" they would be surrendering (possibly), but change the wording to surrender, versus retreat then...