Why is KS a problem?

I started focusing on Lux support recently and I've been on a pretty good winning streak so far, but as it's so difficult to resist the urge to use her fun ulti, what I started seeing is too many people crying about stealing their kills, even if it's just one or two kills.

First of all, I do play all roles from time to time, so I'm not "just" a support, but I never care about who gets the kill as long as we win the game. So, I wonder why so many people cry when their support gets a kill? I mean, I do try to let my adc get a kill when the enemy is closer to our turret than to theirs but very often there will be only a few seconds left to secure a kill and I think it's more of a reflex to try to do everything you can to get them before they step out of the range, even if it means potentially taking the kill with a cc spell. Honestly, it mostly happens by accident, when they get killed by a stun or an auto attack after the stun, and then I get flamed by my adc for taking the kill.

It also happens a lot with people who are from the very start obviously bad. They can't push their lane, they don't try to avoid enemy cc, they dive under turrets with low hp, they are simply hard to support, so in order to actually win the game, I might step up, play more aggressively, and take some kills for the team just so we wouldn't lose the lane. And again, they would start crying as if it was theirs when they pretty much did nothing to get that kill.

Or, in team fights, after the laning phase. God forbid I ulti in the middle of stacked enemies and get some kills... I mean yes, I am playing support, but I'm also playing a very strong mage precisely so I could contribute to winning the game and hold the bot lane in case my adc can't do their job. I can't exactly control how much dmg my stun or slow will do, and if I see the enemy passing their turret with low hp with nobody else near them, am I supposed to NOT ulti them?

Also, as some of you might say, it could influence somebody's grade by the end, but again, it's not like I take every kill. It happens, mostly by accident, and they cry about it way too often. I wouldn't want to do it anyways, since I can't get an S with a support if I have too many kills, but it happens.

And to make it clear, this isn't exactly a rant and I'm not "hurt" by all the flaming, I mostly just ignore them or I do apologise if they don't flame right away. I just found it very interesting, so I am curious as to how you guys see this issue and if it really is that common or simply more related to bad players?