It's a team game. In some cases, my ks will get the team more gold.
At worst, it's a neutral change. Yes, it's true that gold is more important to go on the carries, but it's not like gold is bad for a support. If they don't know for sure you have flash or are in range, etc, it can be worth it to ks from a team perspective.
And, even if it wasn't, let's be real; the grading system takes kills into account, and people are selfish. So sometimes, they're gonna take that kill even when they know they don't need to. That's just people being people.
But when I "ks," it's usually a complete accident. I'm trying to help damage the guy to bring him down faster so we can get more time on an objective, and they happen to crit right before my auto lands. It's a bummer, but it's really not that big a deal either.