Why does breaking alliance with an Ally in War cost stab?

[Piotrzeci]

Breaking alliance with an ally that we are together in a war costs Stability points and I just wonder why.
It doesn't give you anything as far as I know, so why does it have a penalty?

 

[gia257]

is that a stealth insult to friendly ais or what?

 

[atwix]

basically it means you no longer want to come to aid of an ally that might be vulnerable to another attack.

if you want to prevent the dishonor malus and global -5 trust, it could be useful to take the -2 or -1 stab if you got the diplomatic idea group.