The problem with this example is that it is a single event. That one event likely won't make a whole lot of difference in the grand scheme of things. However, as I'll talk about with
@Hriston later, it's all about consistency. Player A is actually playing a consistent character. Player B, without establishing any reason to know that information, is introducing an inconsistency (He knows about trolls when the character has no way of knowing this) into the game, solely to take advantage of the knowledge that the player has.
Now, the troll example has a bajillion workarounds, but, there are legitimate times when the player knows things that the character can't reasonably be expected to know. By using this meta-game knowledge, for the advantage of the character in play, is inconsistent and will drag me right out of being in character.
Because you established these things about your character previously - whether through character generation or through play, and are now being inconsistent to that character.
Of course it applies to everyone at the table. If you're trying to play a horror game and Bob sitting beside you is cracking jokes and completely ignoring the mood and tone of the game, are you saying this won't drag you out of your character?
Me, I set the bar pretty darn low, but, sure, there is a bare minimum of consistency and engagement that I expect from everyone at the table. When that doesn't happen, it pulls me right out of the game and spoils the game for me. When you realize that the player next to you could be replaced with a dice bot and nothing would change at the table, it makes the game very much unfun for me. I make my character with at least a bit of an eye towards making sure I'm going to be entertaining everyone at the table and I expect the same level of basic engagement from everyone else.