I think we need to clarify some terms here to avoid confusion.
The question is not about if a player can decide to attack someone.
In any solution the player can decide weither to attack someone.
The question is what kind of punishment will he get for this.
In the current situation, if someone steals your loot, you get punished for killing the person that stole your loot by loosing rep and getting a murder count.
In my solution, if someone steals your loot you don't get punished for killing the person that stole your loot.
As the player that got his loot stolen, you get less punished for behavior that actually makes sense.
So how is this more restrictive than the current situation?
I understand your point of view, you don't need to clarify yet you definitely don't understand my point of view and the problematics of over-conditioning player behavior.
First of all, in a game such as mortal very rich and with lots of potential in all sorts of player interaction, some people often try and vouch for restrictive type of mechanics that condition gameplay.
Now lets break down this concept of "conditioning player behavior".
What is conditioning players behavior? Basically setting whats right and whats wrong, for example whats wrong is being a criminal and doing criminal actions and whats right is being blue. By having criminal status anyone can attack you without punishment (restrictions).
Why do i consider conditioning player behavior to be something off? Basically because these mechanics tend to narrow down and polarize player behavior into 2 statuses, criminal - blue when theres a whole spectrum of things that happen in-between.
What happens in-between is what makes this game rich, because players can interact beyond the system thats forcing them to necessarily get into a fight.
Now lets set an hypotethical situation. Someone (blue) comes by and loots a (blue) player's mob loot.
Whats happening here is the contesting of a place, the reasons may vary from a previous bad relationship, trying to express domain over the territory, looking for a fight and other things.
What can players do in these situations, players can opt for stablishing a dialogue with the player doing this and solve the problematic or perhaps understand the reason and intentions of why this player is actually doing this. If it doesn't lead to dialogue theres options, to leave the contested place, continue (blue) trying to farm or just to try and murder this player for example.
By the current flagging and punishment system the player who gets criminal status can be attacked by anyone without repercutions. Meaning that these 2 players in this situation won't want to engage combat first unless one or the other is certain may win.
Making mobs loots go blue for players that didn't damage it would for example remove the conflict prior combat and will condition the situation towards one particular thing. The person contesting the place won't be able to loot at all in a variety of situations leading to avoid conflict entirely or just compete on who kills creatures faster. In any case LESS things in-between.
My perspective is the less restrictions the better, let players solve situations on their own instead of forcing them to do something in particular. The law of the world should mostly be dictated by players and not arbitrary conditioning mechanics to polarize their behavior.
This is what i mean "themepark" washed up mechanics. Let players decide how, what and where to do anything instead of restricting them.