I'm seen cases where people have said "I want out of this game" for whatever reason, and they've offered to me (it's happened with both allies and enemies) their SCs. While obviously I've accepted such overtures because they're free SCs, my belief is that ideally someone in that situation someone would surrender so another player can take it on. Otherwise it gives me a significant advantage in the game.
Now, consider this with the scenario in the OP of "throwing the game" as a tactic rather than a way out. It's functionally the same as the situation I just explained, though tactically different.
Both are within the rules, and both involve a human decision which inherently puts it into the spirit of the game. Remember that Diplomacy doesn't require fair or rational thought.