@Blutzer I'll do my best to break it down for you.
Firstly, I will start by saying that the rulebook is intentionally vague to allow admins to utilize admin discretion with more ease and not be prevented from dealing with certain situations or have to deal with extra complaints over some weird technicality in the rules. Many players will try to argue with admin decisions, and the more detailed the rules are the more ways people will find to argue.
For this situation in particular, I took a screenshot of the section of the rules and highlighted what I feel covers why wardens shouldn't place KOS markers inside the game, and keep in mind that the rulebook works in conjunction with admin discretion.
- Games must give all prisoners a chance to fairly participate and compete. - I included this rule because I would argue that placing a KOS marker and denying Ts access to a play area of a map game is not very fair at all. Essentially, all it's doing is making it easier for the CTs to kill the losers while at the same time making the map game more difficult for the Ts. Not only that, but it's not necessary because all the warden needs to do is specify that stacking with other players (heads are inside of each other) in the game is KOS, and there is no reason other than that I can think of that makes it necessary to place a KOS marker in the game. If CTs are unable to determine/shoot the loser's heads individually, it's honestly more of a skill issue and making the game more restricted for Ts due to their own inability to kill the losers is just not fair in my eyes. Not only that, but doing this also gives CTs more of a reason to be toxic and kill needlessly (Ts are trapped in the game regardless, no point in adding more variables to a situation that equates to more toxicity/needless deaths, especially on a map like augment where rounds are 10 mins long and angle vector is quite literally the first game that's played almost every time. Having been a victim to this kind of death personally, it sucked being killed for something that stupid and having to wait out an eternity long augment round, especially when it wasn't even necessary for the warden to do it in the first place.
- Games that are custom (not a map-game) must not be too difficult unless the warden can successfully demonstrate it in no more than 1 attempt. - I included this rule because the moment a warden decides to modify the play area of a map game, he is customizing it and the rules clearly define that any custom game (I know it says customs are not map games, but by adding a marker and restricting the play area of the game, the warden has customized the map game) which makes is subjective to the enforcement of this rule by technicalities. For this reason, even though the rules might not always clearly define the specifics of 1 of 1000s of different situations that can arise in JB, I highly suggest avoiding any kind of alterations to how a map game is supposed to work and be played, because admins could technically enforce this rule and make warden demonstrate a playthrough of the game with the restrictions he has set on it. IMO it would be wayyyyy easier to just make stacking in the game KOS so you can still kill the losers with ease and not have to risk demonstrating/getting Ts needlessly killed, especially since CTs talk over warden all the time and people usually get killed because of not hearing that this marker was placed/KOS.
All in all, I disagree that markers should ever be placed in a game that interferes with how it is played especially when the only point is to make CT lives a little bit easier while simultaneously making the game more difficult for Ts, and gives certain players yet another reason to be toxic and kill needlessly. Some players are new and might still be a bit confused with how orders work and may have had 0 intentions to rebel and just want to play the game, so imagine how they would feel if they got shot over something like this. Furthermore, it's just as easy, if not easier to just make stacking in the game KOS. Idk why people all of a sudden started thinking it's okay to alter map games with a specific design which is likely the way it is for a reason, but just because some people may not have taken action in the past against it, or that it's not explicitly defined in the rules, does not mean that it's okay to do. I can vouch through personal experience that doing this does and will continue to get players killed needlessly, and wardens should try to avoid doing it. If you have any questions/comments/concerns about my standing in regards to this post, feel free to let me know and I will hear you out and try to clear up any potential misunderstandings. Afterwards, if you still have not reached an answer that you feel satisfies this question, I will try to get a couple ATs in on the discussion to further clear it up (1 of the admins that was trying to advise against this action was an AT just fyi).