Yeah, for some reason they don't like that.
I learned that when attempting to loot some dead Flaming Fist at Waukeen's Rest in Act 1.
There was some other Flaming Fist still alive, kneeling beside that dead one's body, obviously rummaging through his stuff.
So i figured it must be all free for taking. It wasn't, but killing her fixed the problem.
But on a more serious note, i run into this problem a few times when i looted what was clearly a dead enemy - one that I even killed myself!
For example, some of the goblins attacking the Emerald Grove - some were ok for me to loot, some were "off limits" and i had to do it sneaky sneaky.
Why would the tiefs/druids care about me taking those gobbo's stuff?
Was it a bug? or is there some logic to this that i am missing?