The point of the feature is not to prevent users from running anything in the background. It's that *anything* the user runs has proper systemd confinement, so it's obvious and manageable by the administrator. Without this feature, the only reliable way to achieve the same thing is to reboot every system.
> This default is nonsense the only thing that it really does is break stuff that relies on processes being executed after the user closes his session. Yes, there's an obscure systemd-run command that only the systemd devs know and can make your programs run forever but what's wrong with "&" or just running "screen" to create a persistent session??
Maybe it's obscure to you, but it's foolish to suggest that it will forever be so. What's wrong with your shell understanding that "&" needs more sophisticated handling than fork/exec* these days? There's no reason why shells can't handle this for you, or you can setup your shell to handle it for you. There's already been discussion about creating wrapper scripts in Fedora for screen and tmux that automatically handle execution via system-run, so I'm unsure what the issue is.