That's quite an overreaction. It's more about keeping up the pressure on both sides of the Empire. Which makes sense when you remember that 5.0 is Garlemald.
Heidrek is a bit removed from the Ala Mhigan struggle so if an Imperial came along and asked for help to kill something bad? He'd go along with it, the politics wouldn't even factor into his decision.
This habit combined with the fact that Final Fantasy is a setting where I want to play EVERYTHING has caused me no end of consternation, I can tell you that.