This never made sense to me, but it falls into the realm of the guys who have been saying that KDA needs to be used to determine promotions in ranked. Too many people think of this game as 5 individual fighters who just happen to be pushing toward a common victory condition and if we work together that's nice too.
My attitude to having a kill stolen has always been, "I'll get the next one." I'm more interested in objectives so a kill that helps the team push for an objective is something I like. I really don't worry about who got it.
(of course, I also have become quite good at farming so one kill doesn't have a huge effect on my gold stockpile. And I do still have good games for kills like a recent one on Sona where I went 6/0/8 including a triple kill without really paying attention - none stolen from my ADC as all occurred after laning was over.)