Description of problem:
Shouldn't in be in the main systemd package?
Version-Release number of selected component (if applicable):
No, this is intended this way: the -units package exists so that packages can depend on it without actually pulling in the main systemd package.
For this to work the units package contains exactly what is is necessary for the %post rpm scripts which install systemd services files to work: the unit directories where to place the units in, a few dependencies and the systemctl tool which is used to remove/create symlinks.