First of all, we have to remember the viev of camped mobs is completly unreal image, no matter how you look at it.
With this in mind i came to a conclussion: Maybe these who don't drop loot are just supposed to be there? Look bad, like they are camping by fire in a place you should stay away if you don't look for trouble.
But why some rogues at arena drop loot and rogues next to them don't, I'm not sure. Maybe for a simple illusion that not every damn rogue is camped.
Dark rogues could drop nice loot, but they drop rings instead. Why do they? because if you are doing a quest, which tell the rogues are in unknown place, you dn't except to find there bunch of campers who fight them for their loot.
I have said once or few already in the past that the looting system is fracking up every other bit of the game, because you can make so much money on it, every other job must be same as profitable to make sense, which is then unrealistic and in the end we have a mess. Something you can enjoy OOCly, but not ICly.