The free attack is the consequence, but it is not the design goal. The design goal is to lock down opponents. The design goal is to make the Fighter sticky. How they achieved that is not the goal, it's the game mechanic to accomplish the goal.
The problem with a forced lockdown is that it is too much like compulsion magic. It appears that the designers prefered a system of choice with consequences.
That's uber metagamey.
"Ignore the other 4 monsters that are attacking us and concentrate on the last monster".
Doesn't matter if its the monsters doing it or the PCs doing it, unless there is some specific goal to it (for example: escort mission where the NPCs try to kill the person being escorted or some such thing).
Is it a good strategy? Does it work? Yes and yes. But its super metagamey. If I'm playing an SRPG, I'd do something like that, but in D&D....well, you might as well just play a wargame if you are going to fight like that.

Not really when you have 50 gp potions of healing. Anyone can pour that down the throat of the leader as easily as the leader can heal someone (well, almost as easily. I figure if you're far enough in a fight that someone get's knocked out, then the leader has prolly already used most of his/her healing)
Choice with consequence was the design goal.
A choice with consequences that's so bad as to never be taken is a non-choice and therefore manifestly not the design goal.
There's a difference between good tactics in a tabletop game and metagame tactics in a tabletop game.
Matagame tactics may be "good" in the sense that they work well, but you're breaking the 4th wall doing that.
And if you are a paladin, or some other type of character with a behavioral code, you very well may use rules not unlike the Marquis of Queensbury Rules in combat.
Others, like Warlords or war trained fighters, may try to follow Sun Tzu and the Art of War.
Others may not have the first clue about fighting as part of a team and using such tactics with those characters would be very metagamey.
To each his own, though, I guess.
Just by using the term "action economy" you are showing that you view combat in D&D as being completely separate from the roleplaying. You are looking at the combat as if it is just a war game; as if you were playing Warhammer or something.
That works just fine, if your goal is to win every battle using the smallest amount of resources possible, but you aren't really playing your character any more; he's just a piece on a chess board now with no feelings or goals.
And if the Striker is your friend, then getting him back conscious is probably urgent.
KarinsDad: I don't understand how you can argue that point, and then question the decisions of a DM that's opening himself to AoO's. If it makes more sense for the NPC's to swarm single targets, and the Fighter isn't a good choice always, then it's perfectly understandable why they might be willing to risk AoO's to get the Leader. As you yourself said, it's much more costly to heal the Leader when they fall than for the Leader to heal, say, the Fighter.The best situation for the NPCs is if multiple NPCs attack a single PC. Granted, the Fighter is not often the best target for this (typically the Leader is)
There's a difference between good tactics in a tabletop game and metagame tactics in a tabletop game.
Matagame tactics may be "good" in the sense that they work well, but you're breaking the 4th wall doing that.
And if you are a paladin, or some other type of character with a behavioral code, you very well may use rules not unlike the Marquis of Queensbury Rules in combat.
Others, like Warlords or war trained fighters, may try to follow Sun Tzu and the Art of War.
Others may not have the first clue about fighting as part of a team and using such tactics with those characters would be very metagamey.
To each his own, though, I guess.
Just by using the term "action economy" you are showing that you view combat in D&D as being completely separate from the roleplaying. You are looking at the combat as if it is just a war game; as if you were playing Warhammer or something.
That works just fine, if your goal is to win every battle using the smallest amount of resources possible, but you aren't really playing your character any more; he's just a piece on a chess board now with no feelings or goals.
And if the Striker is your friend, then getting him back conscious is probably urgent.
KarinsDad: I don't understand how you can argue that point, and then question the decisions of a DM that's opening himself to AoO's. If it makes more sense for the NPC's to swarm single targets, and the Fighter isn't a good choice always, then it's perfectly understandable why they might be willing to risk AoO's to get the Leader. As you yourself said, it's much more costly to heal the Leader when they fall than for the Leader to heal, say, the Fighter.
For example, one of the Fighter's class skills is Heal. Since Fighters often had decent Wisdom scores, it's a good skill to have. If the NPC's knock down the Leader then the Fighter might end up being the one trying to get back to them to heal them. This would mean that now the Fighter would most likely be provoking AoO's, which would even the odds considering the AoO's that the NPC's had to take to get past him, and then he would be blowing a standard action to use Heal on the Leader. If he failed the check, then he might even have to blow another standard action to try again! This is great action economy because the Fighter is now hurt, the Leader is down and the Fighter is blowing standard actions to heal the Leader rather than fighting and marking enemies.
Zombies and Goblins might not go this route, but it's perfectly justifiable for a more intelligent enemy like a Hobgoblin (or Goblins led by a Hobgoblin) to do something like this. So it might not happen every encounter but it's bound to come up when dealing with more intelligent enemies that would be more versed in battlefield tactics.
Attacking a bloodied Leader might be one of those times, situation depending. But just giving the PC Fighter free OAs in order to get to the "squishier" targets in the back on round one typically does not make sense. Swarming the closest target makes more sense usually. And if foes want to get to the squishier targets, they should just go around the Fighter in front on round one, not stop and engage.
Actually, the opponent needs to know that a Leader is a Leader to make this type of decision.
That doesn't typically happen until later in combat.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.