In one of my current games, a Union force is defending Memphis from a CSA attempt to re-take it. If we tie and an * result is rolled, does the Union win the tie? I can't find anywhere in the rules that suggest the defending benefit of a resource space is limited to the CSA only, but it seems a bit odd that the USA would defend better when fighting in such a place.
If the rule does indeed apply to both sides, what is the historical underpinning for it for the Union?
All sides benefit from defending in a resource space. Look at the battle of Nashville, the advantage lies in the defense of a location with some level of defense. Does it matter who built the defenses and who mans them, if you are in the defenses you benefit.