It is a Chalenge.
Don't think of it as denied kills for you. Proximity kills don't get you anything anyway. Think of it as three deaths for the bomber guy and instead of landing his sortie for 100% of his points, he only collects 25% of his accumulated points for that sortie (if he damaged anything, of course). So basically he flew all that way and wasted all that time for very little gain.
If you get altitude and position yourself between the bomber and his target, he'll be less inclined to bail. Killing a bomber BEFORE he gets to the target is the ultimate win. He doesn't damage the target. He gets three deaths, you get three kills and if you land them, it's 100% points for you and WTG's all around.