Well, it entirely depends on the context of the kill. In CoD, a great deal of emphasis is placed by a lot of the players on scoring kills, and from what I know about the game having not actually played it, people tend to focus on K/D ratios above all. Their objective is to maximise their K/D, so anyone who gets in the way of that really pisses them off.
For Tribes, there's much more focus on points rather than just kills, which you'll get for loads of things. I've managed to top scoreboards by placing my focus exclusively on capping and going fast, which I'm informed one has got to do, so I really don't care if people take my kills off me, and with the amount of points that can be awarded for destroying secondary objectives, that's probably why nobody else seems to care. Their objective is points, not kills.
As for LoL, I almost exclusively play support (6 deaths to 34 assists ftw), so I know about the damage that killstealing can do. A kill grants a large (ish) amount of gold and experience to the killer, and around half the reward to all assistants, depending on how much they did to help. When some members of the team rely heavily on gold and/or experience to succeed, whilst others (like me) don't really benefit as much, it's quite understandable that people would be annoyed when you score a kill in their place.