I play a strategy game called atwar and there are basic rules of conduct that most players agree on. Specifically, when you make an alliance it is assumed that the alliance will be kept the remainder of the game. Mostly it is noobs that break alliances but there's also people who knowingly abuse this rule. I think it's 100% cheating to do this because it requires zero skill and most players will automatically give you the benefit of the doubt. It is completely against the spirit of the game and will quickly earn you a bad reputation.
Haven is obviously not a perfect comparison but it's worth thinking about.