The fact that `KillUserProcess` is default to on is not something that systemd forces to your distro, there is even a flag to disable it at build time. It does makes sense in many situations (like IoT, embedded, etc.), that is why it is on, but distro mantainers also needs to know what is better for its users (that anyway is a really small part of Linux users, embedded is much bigger).

"Not in upstream" doesn't seem to correspond with the comment you replied to.