Arch tries to generally avoid changing "default" behavior. Systemd doesn't automatically do that, so Arch won't ship systemd like that. Arch also generally avoids shipping with timers enabled.
A little of column A, a little of column B. Arch is very much a distro developed for the packagers. OTOH, as someone who often has to dive into the details regardless of distro, I generally appreciate the plain-vanilla approach to arch's packaging (since I can just set the package up as opposed to undoing whatever helpful defaults and changes Debian's decided to add).
I personally (using Arch, btw) definitely prefer Arch's behaviour. If I update or install a daemon, I generally want to configure it before (re)starting it.
Arch does reload service files (not user ones though) IIRC. I also disagree with GP, I don't reboot, I use needsrestart and/or my own logic (simple grep for "(deleted)" within proc maps and then show cmdline).
Makes plenty of sense to me.
90% of the scripts are to do that, or to recompile .pyc files when you install a new module or when you install a new version of python.