I realize that it's not against the rules. Otherwise I would have been more outraged, but it doesn't change the fact that it put me in a very compromising situation. And instead of leaving supply centres open on purpose, why not CD and make other people work for supply centres. Not to mention give someone else who might want to take over for you a chance to play.
I'm not trying to insult this player, who shall remain nameless, I am merely suggesting that this was unfair due to the fact that I had no shot at any of those supply centres, and therefore it put me at an immediate disadvantage, even moreso than I was already facing. Now that countries that I am having conflict with have more centres and therefore more builds, it has basically ruined any chance I might have had at being a legitimate threat in the game. As for the alliance, it wasn't so much that I was allied with this person as it was that they were warring with players that I was also having conflicts with, which means that they took some of the focus off of going after me. Now that the distraction is not there for the people who want to wipe me off the face of the map, they will be able to focus on going after me instead.
Regarding EdiBirsan's comment, I can understand the point of your argument, and certainly will not dispute your opinion, being that you are a highly experienced and skilled member of the Diplomacy community. Far be it from me to argue with someone who has far more Diplomacy savvy than I ever could hope to or will have. However, regardless of the strategic merits of doing such a thing, I find it immoral to do something that is not fair to ALL parties involved in the game.
I suppose I do not understand this move because I try to stick it out to the bitter end in games that I am losing, because I enter games to play them and to have fun, not to leave when I feel things are not going my way, and especially not in that manner. I just feel that going CD and letting someone else take over for you is the better thing to do, as it does not put people at so much of a disadvantage.
Perhaps there should be some sort of way that you can voluntarily civil disorder to leave a game, while simultaneously alerting all interested players that there is a new game they can join. That way, someone else can pick up right where they left off, and the game can run it's natural course, instead of there being easily obtainable supply centres available when they normally would not be so.