The thread shouldn't be about why "Kill stealing isn't a problem" and more about "Why our definition of KSing is overbroad and a bad thing"
Real, Actual KSing is a sign of poor sportsmanship, bad play, and puts teammates at risk in situations that they shouldn't have been at risk in at all. It expends resources (Time, HP, spells, Mana) and opens the rest of your team up to bad situations (Zerging to bot to 'secure' a kill inside of Nunu's ult just opened your lane up to attack, and if bot was a 2v1 turned 3v1, top is now exposed to a possible 4v2.
What IS Ksing:
Autoattacking as an AP carry until the target is at low health then burst-comboing for the last 2%
Global-ulting a solidified gank (a 2v1 with heavy CC)
Entering a 2v1 (in your favour) by firing a ranged poke through your allies (when the target is rooted/snared/stunned and an execute is imminent) to snag the kill. If they're going to flash and try and escape you can still cut them off in two seconds.
Urchin Striking into a 2v1 (their favour), landing the kill on one, then abandoning the other target to let them mop up your ally (who spent most of his time on the guy you just nuked)
WHAT ISN'T A KS:
Kog-ulting the runners for the last hit
Damage over time effects
Autoattacks if you've been engaged in the fight
Global Ults that are -called for in chat-
Twitch's Expunge (YMMV)
Sniping a runner with ANYTHING.