Wicht
Hero
I believe he's talking about who the players view as antagonists, not who the DM chooses to introduce into the game. For instance at character creation the Ranger chooses his "favored enemy" which indicates, to a pretty large extent that any creature of that sort will be viewed as an antagonist (although the actual antagonist title may be reserved for the biggest bad of the selected creature type). The DM certainly does create and introduce the monsters, but that's different than creating antagonists. I've had plenty of players ignore Big Bads and the like who were introduced as potential antagonists for the players. They have done the same to helpful NPCs. The DM can only introduce creatures, not whether those creatures are antagonists. Hopefully the DM and players are in sync enough that the DM is only introducing potential antagonists that will be viewed the same way by the PCs. Ideally anyway. The question is though, where's it written who gets to label the creature an antagonist? Who gets to say, this creature has meaning beyond the simple encounter? Even if the DM continually reintroduces the same creature, that doesn't mean the players will respond or even particularly care about that creature (thus elevating it to the status of antagonist). Is that the DM failing or the players not playing right, according the rules? I have no idea, but I think that's what he's talking about.
An antagonist is someone threating, or acting against the interest of the PCs. If the PCs want to antagonize the king, then I guess they can choose to act as if a protagonist was against them.
And if they choose to think the wolf eating their faces is a friend, then I can't help them.
But the DM is under no obligation to make a character friendly if he does not want to. He gets the final say in who is actually acting against the PCs, regardless of their perception.