Ok ok, I'm not afraid to say that I was wrong. I found something that seems to approve of what you said. It was posted on November 18, 2013 on the official Elder Scrolls website. Did it change since then? I'm going to assume it didn't. So you were right.
From "Ask Us Anything: Variety Pack 11"
I've played many multi-player games, and 99% of them are eventually plagued with the same enjoyment-killing problem. What is this problem? Kill-stealing. You know, jerks who like to go into areas where people are farming or trying to level and kill-steal them relentlessly. Does ESO have a plan to implement something to prevent this? – Emily
Fighting the enemies of your alliance—NPCs as well as PCs—should be something every aspiring hero jumps at the opportunity to do. We don’t intend to limit helpful behavior in Tamriel by lowering the experience or loot you can take away from a hard-fought victory. If you help someone kill an enemy he is already engaged with, all you can do is take away some of the pride he might have felt by winning the fight on his own. He’ll still receive the full amount of experience and loot. If you did enough damage (or provided enough support), you might get some experience and loot of your own, too. And if the two of you decide to group together, both of you will get even more…
Sorry for saying something false, you were right, but my signature tells it