I posted a thread like this once upon a time when I learned that not everyone has the same priorities. I find it's very common in all variations of the game (PPSC & WTA, slow & live, full press &gunboat, online & face-to-face) that not everyone cares about winning, drawing, or even surviving. Sometimes, players are more motivated by revenge than by the possibility of a win or draw. I was recently at a tournament where a player threw a game, not only allowing but also assisting another player to solo, motivated purely by a desire to "teach a lesson" to someone who'd stabbed him. I recently played a high-stakes WTA game online in which the exact same thing happened. Neither of these players used throwing the game as strategy to prompt those attacking him to stop; even after a ceasefire and several seasons of supports and other favorable moves, the tanking players continued to tank. Like you,Thucy, I was, to say the least, dismayed, and I was not alone. In the tourney situation, one player even quit the game out of disgust. However, although I don't like it, I've made my peace with players' throwing games.
You see, Diplomacy, by virtue of its interpersonal nature, is a complicated game. Players are complex bundles of motivations, proficiencies, experiences, baggage, and neuroses, and they bring these myriad influences with them to the table (or chatroom). A skillful Diplomacy player must, therefore, be able to size up her/his fellow players and know how they will react to various stimuli, especially stabs. The predisposition of a player to throw or merely quit a game is no more or less a trait to be factored into interactions with her/him than that player's predispositions to lie, stab, throw a tantrum, or miss a turn. Thus, a player's throwing a game is not extraordinary; it's just another unfortunate consequence to be avoided in a game, another concern to influence the difficult decisions a Diplomacy player must make.