I'm sorry, under what scenario is this happening, again?
Sorry, as you note, that should be when they've readied an action to charge at the start of the defender's next turn.
The reason for the other character to prefer ranged combat, regardless of general preferences (what, no one ever exploits Sling Expertise for all it's worth?), only goes to present a case in which two enemies can ignore two members of your party with little risk...when it's one party member's turn.
So, the striker in question gets one OA, not two.
So, by readying an action, these two enemies reduce the total number of opportunity actions available to the party from four to one? That's not so bad if they're losing out on a really good attack they can't perform on a charge (though this tends to hurt monsters less than player characters), but they could always just focus fire on the non-defender with their best attacks instead. Is the -2 penalty on their attack rolls enough compensation to the defender who can't attack because it's his turn?
How about artillery or controllers that can gleefully ignore the guy standing right beside them to target people anywhere else on the battlefield? It's enough to make a defender player join the ranks of those that only play strikers.
Playing metagames with initiative actions /can/ work, and it can even make sense from a non-metagame perspecitve, but doing so systematically can become predictable, and end up costing a lot in the action economy.
Maybe, but if it can consistently shut down a defender's best options, rather than those of the creatures the defender is trying to contain, who suffers the most?
Consider the generic fighter. This sort of stunt completely negates the option to use combat superiority against an opponent that opts to charge away from you. It means they can attack the guy standing right next to you without any fear of you hitting them back, even though that's supposed to be one of your tricks. The warden and generic paladin do a bit better; the battlemind and swordmage a lot worse, and anyone with a defender aura active is left wondering why they bother.
Again, if this strategy for opponents only applied in borderline cases, it might not be so bad, but a DM can reliably utilize this strategy against defenders as the rule, rather than the exception.