I think this is more like adding lines to /etc/profile.d or .bashrc than adding a service. Linux has a pretty complicated flow if you want to know what runs on login or on startup.
If the application choses to do so. Installation used to require root and I don't see a way who root could prevented from be enabling a systemd service. Short of having a signed read-only rootfs which no common distro has.
Nowadays you don't even need root to install something that runs a every login: There is the user instance of systemd. Of course without root those services are limited in what they can do.