I understand the basic idea of zone of control, but I was surprised that, on the main map, runners were unaffected.  Why?  Is this special to them, like cavalry?  Is zone of control automatically cancelled by difference in speed?

As far as I'm aware, speed doesn't have an effect. It may be a hidden benefit to runners that isn't called out in the tooltips.

Okay, I started a new test game specifically to check this out by choosing the Harappans.  It appears to me that when a runner starts its turn adjacent to an enemy, it can move two spaces away rather than the usual one.  But as far as I can see, the rest of the zone of control rule is working as usual -- it's not live cavalry.  

Seems like this should be documented in-game.