The same logic applies to any game.
Can you make truces in Risk and team up against another person?
Can you make unfair trades in Monopoly moments before you go bankrupt, just to help the underdog?
Can you fold in poker in a big hand so the person who was modest to you today gets more chips?
Etc.
There is no rule against this style of game play, it just makes for dumb games.
There is a special place in...for anyone who breaks the game code of playing within in spirit of the actual game.
There are the rules of a specific game, and there is the social contract of game-playing;
any given action can violate one, both, or neither. I was saying that the rules of Catan were
not being followed, not that this was not how Catan (or games in general) should be played.
There are 4 categories of actions relating to a game:
1. Actions defined by the rules and which create a change in the game state. (Example: Moving a piece according to the rules.)
2. Actions not defined by the rules and which create a change in the game state. (Examples: Moving a piece not according to the rules, flipping the table.)
3. Actions not defined by the rules which do not create a change in the game state and have no influence on the game. (Example: Eating pizza.)
4. Actions not defined by the rules which do not create a change in the game state, but nevertheless have influence on the game. (Example: Deal-making, leaving.)
(Note that the Risk example quoted was in category 4, while the others were in category 1.)
A game's rules usually address only category 1 (although in some cases, such as multiplayer conflict games,
like Risk, or "social deduction" games, like Mafia/Werewolf, games may be designed around category-4 actions
being a part of the experience and they may be mentioned, though not limited by a definition, in the rules).
The social contract of game-playing has two aspects. One is the allowance of some categories of actions and not others,
while the other is the "spirit of the game," usually including the idea that each player will do their best to win the game
themselves or come as close to doing so as possible (and not try to cause a specific other player to win, known among
game players as "king-making"). How the social contract treats category-4 actions will vary by group and by specific game.