Ya I actually noticed that in Tribes too, but I think it has to do with how everything is scored. In CoD and in LoL, kills can make a big difference. In CoD getting a kill means you're one step closer to that game breaking killstreak, while in LoL, a kill means both a level and gold advantage which often leads to an easier game for you. In LoL I don't think killstealing is as prevelant at higher levels, cause you will get the assist points, plus the main benefit is that person is out of the game for the time until they respawn, but I'll admit to getting angry if I finish a person off and am about to kill them and someone wastes their ult to take the kill from across the map when there was no chance they would get away.
In tribes its a lot different. A kill is not even worth that much, and there is a stigma against what I think is called "deuling in the midfield", since those people are generally seen to be no or little help to a team. If I'm having a good game (INF player), I'll usually place in the top 5 of my team, but I almost never have the most kills. I get a lot of points from assists and gen destruction.