The maintainer script calls systemctl directly. Actions such as enabling a unit file should be done using deb-systemd-helper so that they work on machines with or without systemd. Starting a service should be done via invoke-rc.d if the service has a corresponding sysvinit script or deb-systemd-invoke if it does not.
If you are using debhelper, please use the systemd debhelper addon, which is provided by debhelper (>= 9.20160709~) | dh-systemd.
Refer to https://wiki.debian.org/Teams/pkg-systemd/Packaging for details.