A "practice" implies that it is common. It's not. It's rare. There are very few DMs who will engage in trying to control every aspect of what the players want to do
Let's say that it's common enough of a phenomenon to warrant the creation of a moderately-circulated pejorative to describe it and a 900+ thread of people debating the scope of its applicability.
The social contract is stronger than any rule system.
Maybe if I remind you of this post often enough, then maybe your words will transform into an actual conviction that you put into practice:
In your opinion. You shouldn't be presenting your opinion as if it were fact, because it's not.
Extraordinary claims require extraordinary evidence.
5e tells the DM to discourage metagame thinking.
It depends on how you read the imperative. Is it prohibitive? Commanding? Instructive? Advisory? Suggestive? Nowhere though does it explicitly say that metagaming is disallowed. Discouraging a thing is not prohibiting a thing. We see this, for example, in "Discourage swearing by establishing a swear jar," which does not prohibit swearing or imply illegality. And if we are in a system that lives by "rulings not rules," then what extent can we say that metagaming is illegal in the rules?
Furthermore, discouraging a thing does not make the thing illegal or cheating. The book may note how a GM would possibly want to discourage the creation of a chaotic evil PC among a bunch of Lawful Good PCs due to disruption it may cause the game. But in itself, it is not prohibited.
And along similar lines, discouraging a thing does not inherently make it an invalid part of play. Fouling is technically illegal in basketball. There are explicit rules to discourage excessive fouling and what may be regarded as unsportsmanlike conduct. However, fouling is an intentional and strategic part of how basketball and many other sports (such as football/soccer) are played. Players are even trained about how to foul other players and how to draw fouls. It's almost as if the metagame is a fundamental part of the game.
But let's also consider this. I am a player reading the player's handbook, which tells me, the player, how to play the game. As a humble player, I do not have access to the DMG, because I am not the Dungeon Master. Where is my prohibition against metagaming in the PHB? If this is a player issue, then why is this issue of "foul play" entirely absent in the player's manual where a player, who would obviously need to know of its illegality as part of play, could find it?
All that said, I'm of a similar mind as the Angry GM when it comes to metagaming. Metagaming is more often than not a symptom that the social contract already broke down between players and the GM, and it's not always the player's fault:
"Dear GMs: Metagaming is YOUR Fault."
BTW, Max, I am still awaiting your alternative term for Mother-May-I that you think would be more suitable to describe the practice. If you don't like the term "practice," for reasons you have already provided, then substitute "practice" with "this particular aspect of dysfunctional play that jerk DMs do." You have definitely had more than a second of thought to come up with one by this point.
Edit:
So you could just call it "GM decides"? I would think that would be a lot more neutral terminology.
Dear [MENTION=23751]Maxperson[/MENTION]: This is an example of a person providing an alternative term. We can of course debate the merits of this alternative term, but they have nevertheless proposed one, and that should be lauded.
I was wondering if Ovinomancer was saying that the GM saying "No, Roll to hit" would be Denial of "I Kill the Orc" and thus "Mother May I". (Frankly I'm still wondering).
GM: "Let's roll to find out."
It's not as if the GMing principle is called "Say yes or it's Mother-May-I." Please remember that a critical part of SYORTD lies in the second part of the phrase: "or roll the dice."