Since it was first added to the game.
Neither does the description for Broken Crown, but they both work in the same manner. I would also venture a guess that Cains works this way as well, but I have not tested it.
The range is fairly large. About 1 1/2 screens. It is unlikely that you will kill a mob and be more than 1 1/2 screens away before the DBs drop. However, if you are playing something like a hydra build or Marauders, then this might happen if your minions kill the elite offscreen.
If you have a suggestion, then it belongs in another forum because they do not take game suggestions from the Bug report forum.
Please read: How to Write a Good Bug Report
Feedback is NOT a bug. Anything related to feedback, class balance, boss suggestions, etc. belongs in another forum.
Again, this is not a bug. If it were a bug it would not have been introduced into the game like this several years ago and then never changed.