It seems like a bug. Same thing is for enlisting a corporation or the whole alliance into FW. For some reason having positive standing between alliance and imperial faction is not enough for alliance to join the FW. Also I have an open ticket with the described problem via EVE Support but have no official response yet. So the question is still stands the same: What are the exact requirements for a character/corporation/alliance to join Factional Warfare.
Even if it should be like what you say, but it doesn’t work that way. My alliance has a positive unmodified standings towards one of the imperial faction and we are not able to join this faction side in FW because it says that the standing is negative. Same thing as Sheriff described. It is most likely the UI bug.
I saw nothing about standings for an alliance to join fw. Maybe its one of the corps causing issues with its rolling average against one of the main 4 empires.
The CEO or Director of a corporation can enlist their entire corporation (i.e. all their members) to Factional Warfare, and the executor of an alliance may do so on behalf of the alliance. If part of an alliance the corporation retains the decision to join Faction Warfare, so an Alliance can not prevent a member corporation from joining.
Unless something changed, alliances do not have standings with npcs. Only pilots and corps do.
Well in our case it seems to be that some our claimed systems in Null Sec region don’t allow us to be a part of FW. But it is not officially clarified by GMs or CCP, just personal guess. I will keep investigating the issue.