First of all, you don't have to COMPLAIN about a kill steal to point it out.
Second of all, kill stealing is more of a matter in MOBA games then in games such as Halo, because while FPSes reward you for kills in the long run, kills are more of a short term thing in MOBAs, where, most of the time, you need kills to do well in a single game. Kill stealing in LoL, for example, is a case when a champion (mainly supports) who do not need kills to do well in the match take kills from teammates who NEED the kills to do well in the game.
Just before I go any further, let me just say that in LoL, kills give you gold, which you buy items with to become more powerful, similar to how enemy minions give you farm; just not as much. Kills generally give you much more gold than say, a few creeps, and they also give you experience, more so than said creeps, to level up with and power up your skills with. This is why, to you people who have never heard of LoL, kills are so important compared to say, in CoD.
Example Case:
Ashe and Leona go and unleash their attacks on the enemy Graves.
Leona finishes Graves off with an E, meaning that Ashe does not get the kill.
Ashe needs farm and kills to excel later in the game (same reason why supports are yelled at for killing creeps), while Leona is a support, and is meant to instead HELP others such as Ashe get kills.
And that's why people complain about it... but unfortunately, this is the LoL community, where complaints are either taken way too offensively or are meant offensively. While I disagree on your matter of how kill steals matter, there are many things I agree with you on. I'd agree with about 80% of what you're saying, only because the matter isn't just limited to 100% deliberately stolen by someone who ONLY took the last hit.
For starters, there's no point COMPLAINING about kill stealing. That'll only lower morale and start arguments, resulting in the end being a typical "bg report Sona for bad support flame and ks". POINTING it out, on the other hand, is a way to show somebody what they have done wrong, so they'll keep it in mind and instead be like "k thnx won't do it again". Presto, you've helped another teammate, and you might even get a Helpful Honour.
Second of all, and this is something that really gets my piss into a boil, is the VIEWING of a kill steal. Let's take the example case from earilier and modify it a bit:
Ugh noob Leona, you took my kill!
Ashe, I had to E him, he was too far away from you and he was under his turret!
This isn't really a case of kill stealing, but rather finishing the job. Ashe (well, saying that her ultimate was on cooldown), could not reach Graves and slay him, so Leona just finished him off, since she was in range. Ashe still gets an assist for this, which is better than nothing. Unfortunately, this case of viewpoint isn't limited to "this guy needs kill more than you do". Here's another example:
Ezreal and Morgana both have 6 kills, and have just finished off an enemy Warwick, which Morgana gets the kill for.
Ezreal: Noob Morgana ks me!
Morgana: Ezreal, we were both equal in kills, why does it even matter.
Ezreal: Because I'm more awesome and deserve it more, report Morg pls
This Ezreal is clearly a retard. Both him and Morgana benefit from kills, and heck, even if they weren't on equal kills it wouldn't really matter. Ok, it kinda would matter if Morgana had like, 9 kills and Ezreal had about 1. But then the Ezreal should just ask Morgana to let him have future kills in a polite way, and then they progress to both get fed. Presto, yet again you help out someone to play the game.
What I'm trying to say is, kill stealing is sometimes kill stealing, and other times it's just some random shouting about how a Karthus couldn't requiem an enemy because some Riven sliced him in half. That, and kill stealing is a pretty important matter in MOBAs if it IS actually kill stealing.