8.43 says friendly units negate enemy ZOC for retreat and supply only. So friendly ZOC doesn't have any such effect at all, only units?
A Zone of Control is a function of a unit per rule 8.41.
Zones of Control extend into all adjacent hexes per rule 8.42 (with exceptions noted).
Rule 8.43 covers the movement of friendly units and enemy Zones of Control in three instances: a friendly unit must stop when entering a hex that is in an enemy Zone of Control; a friendly unit may not "side slip" between two hexes that are both in an enemy Zone of Control; friendly units that are retreating may retreat through hexes that have at least one friendly unit that is within an enemy Zone of Control, and they may trace supply through a hex with a friendly unit that is in an Enemy Zone of Control.
This final instance of retreating is important because rule 9.46, final bullet on the left hand column, prohibits retreat through an enemy Zone of Control.