Discussing this with my son brought out an interesting point. If I am allowed to cross Johnny, I could make hundreds of troops, move them towards the west to attack "our" enemy, only to break the treaty after I get set up the way I wanted. Yes, there is a 21 day break, but I could be so entrenched that Johnny couldn't possible build a defense in that time.
I know that this isn't necessarily what would happen but...
If a route is laid out for them to traverse, then you can control where the Frenemy can go and defenses can be build as a precaution/deterrence.
As for the Air Force, I was under the impression that one could fly in friendly air unmolested.