AFAIK, the rpmnew is added only to configuration files (e.g. files below /etc), not to system/package files, and the systemd files are not considered configuration files.
Below a excerpt from a web page about systemd:
If you wish to modify the way that a unit functions, the best location to do so is within the/etc/systemd/systemdirectory. Unit files found in this directory location take precedence over any of the other locations on the filesystem. If you need to modify the system's copy of a unit file, putting a replacement in this directory is the safest and most flexible way to do this.