If you give people a hard time for "stealing kills", all you're going to do is make it so the next time you need help with a kill, that person that you gave a hard time, will, if they care at all, lay off, try to let you have your precious kill, and you'll probably fuck it up.
I'd rather all of the players on my team be sharks. Sure, there's strategy involved in LoL, for things like Carry's needing gold, but really, it's better for the support to know that they CAN get in there and help secure a kill. The difference between an assist and a kill is 150 gold, which is both a lot, and not very much. 150 gold goes further on a Carry, but it doesn't completely go to waste on a Support - they need to buy Wards, as well as other things.
The point is, don't worry about it so much. Be happy that you're on a team with tenacious people that want nothing more then to stomp the hell out of their opponent.
I can't speak for Tribes, but seriously, life is far too short to cry about KS'ing in general. In LoL, death is so much more painful to a team then any one kill. Having a team mate down hurts the opposing team far worse then it bolstered anyone in your team. Just be glad one of them is dead, hopefully without losing one of your guys. They send your support to the hospital, you send their jungler to the morgue.
It's a gut human reaction to feel that something was stolen from you when someone kills your target, but you need to get past that. Only the noobs really complain about KS'ing. Pros will pick a kill apart, see how it could have served their team better, but I don't think many of them would be like "that guy totally stole that guy's kill". It's a team game, and things can be done better, but live together, die alone.
and that's a lot of allusions. I think I need sleep.