Shaman may have the power of magic (or maybe not), but does that auto-mean the thinly clad bespectacled youth hiding behind the snorting ½-orc can wield such power? And given that the shaman may use divine magic & wear armour, where then is the auto-wizard logo?
The logo is when he makes the strange noises and starts to cast a spell. In older editions of D&D (BECMI, AD&D) you declare spell casting at the beginning of the round and then roll initative. Any hit disrupts the casting and fighters do not get opportunity attacks.
In a world where most savage humanoid tribes have casters (and have an INT of 9+) then it is reasonable that they can guess that the guy casting an elaborate spell is bad news. Now, it is true that less informed opponents might not be able to figure out if the cleric casting Cure Light Wounds or the Magic User casting Sleep is larger threat.
Of course, game worlds may vary and any specific setting might have magic be exceedingly rare. But in the rules you can find magic users and clerics in both hamlets and small bands of humanoids.
It's counter-intuitive if you are used to 3E/4E, but spell disruption is easy in older editions and there are no mechanisms for casting quietly or without gestures (if the spell requires them as components).
Ah yes, but what I was concerned about was the auto-javelin into a PC merely because he was ill-armoured, and inspite of there being other more immediate, threatening targets .
Secondly, I think another point should be that the DM in earlier editions was expected to have a plan of the antogonists actions PRIOR to the player declaring their intentions. If the wizard player hangs back for another round, fidgeting with his backpack, is there any reason he is suddenly designated "the primary target"? It doesn't make sense. Sure,
if he starts casting a spell,
and there are combatants not otherwise engaged in melee, they can try to harm the spellcaster before the spell goes off (initiative + casting time).
Otherwise, there had better be a better reason to start making the wizard's life miserable other than a kobold observing:
"Hey, dat fella in da back dere aint go no proper kit on! I'm gonna try snuff 'im while dat orc-breed tries ta bash ma brain in."