This exactly. If a player is playing the game in good faith it shouldn't matter if they're metagaming or not. If you spoil the movie we're watching, it doesn't matter if you've seen the movie before or not, your specific behavior has ruined the experience.I think that's part of it.
I also think that this goes back to the DM being at fault. They're presenting the players with a challenge... if the challenge is such that things the players already know gives them an unfair advantage, then that's a flawed challenge. Come up with another way to challenge the players.
Expecting them to pretend not to know the thing they know... what does that challenge? Their ability to play dumb?
It's just silly.
Also, specifically to RPGs, it's really on how the DM runs the game if metagaming (and not being a dick) can ruin it or not.