ssoren, you deftly answered your own question, I think. Spite is the identified reason and motive. But "why" questions are usually rhetorical in nature, I suppose.
So, if spite is the motivation to help another player solo, thus throwing the game, is it (spite) inherent to Diplomacy as a game, or is it outside the bounds of good sportsmanship?
If only for the sake of argument, I think it would be wrong to throw it out without proper examination. So, Diplomacy is a game about domination through maneuver within strict rules of movement, like chess. And it is also a game of communication and relationship, hence the name of the game. As we all know, the social side of the game, involve players who, intentionally or not, draw out good and bad emotions in their fellow players through their actions in the game. When intentional, this is used to manipulate the actions of other players. Additionally, we find there are emotional reactions that are not intended or wished for and cause another player to act against one's interests, as when you former ally helps your opponent solo. I suppose a good player would anticipate that is one possible result of a stab. And if you know this about your opponent, it is simply another factor to weigh in your decisions.
Just a thought.