Or, from the perspective of someone not "in the know"....
1) Madina sells food to D'Hara, for which D'Hara is always in desperate need. So, Madina and D'hara are friendly, and D'Hara might be willing to overlook a minor discretion that doesn't affect them directly and they aren't in any danger of being punished for in order to curry favor with an important food supplier locked in a civil war
2) Barca is an SA nation (I believe - working from that assumption In Game). D'Hara is not. They are obviously friendly, but not exactly joined at the hip. Why does D'Hara care if a Madinan noble passes through D'Hara on his way to Barca to do whatever the hell it is he's doing, as long as it isn't attacking Barca?
3) Aurvandil just took a townsland right on Barca's border. Aurvandil isn't SA, nor are they any sort of Ally. Perhaps Barca isn't too pleased with that? Perhaps Barca doesn't like the idea of a strong Aurvandil on their southern border. Perhaps if a Madinan were to slip through and attack Aurvandil, Barca's reaction might be more of a "Oh, gee! that's terrible! We'll tighten up our border from now on, and send an official protest" -- wink, wink, nudge, nudge.
4) Maybe the Madinan doesn't want to sit and argue the viability of traveling all that way with his Council, so since he's an infil who operates on his own anyway, he takes his men around, loots Argyl and sends back a report saying, "Look how easy. I've broken the stalemate. Send 3000cs." and all of his bored-stupid realm mates who have been guarding the Tower for a month think he's the Cat's Pajamas now.
See how STUPID it might look from someone who isn't so privileged as you as to actually know what's going on in the private diplomacy channels?