Look at any of the current nagios packages and you will see it's shipping a sysvinit script instead of the actual systemd unit it should be. %if 0%{?el7}%{?fc20}%{?fc21}%{?fc22}%{?fc23}%{?fc24} is the conditional, but it doesn't work. Since el5/el6 are the only non systemd releases anymore, might just change this to only match on them and default to systemd unit on.
nagios-4.0.8-4.fc25 has been submitted as an update to Fedora 25. https://bodhi.fedoraproject.org/updates/FEDORA-2016-52c039b6d0
nagios-4.0.8-4.fc25 has been pushed to the Fedora 25 testing repository. If problems still persist, please make note of it in this bug report. See https://fedoraproject.org/wiki/QA:Updates_Testing for instructions on how to install test updates. You can provide feedback for this update here: https://bodhi.fedoraproject.org/updates/FEDORA-2016-52c039b6d0
nagios-4.0.8-4.fc25 has been pushed to the Fedora 25 stable repository. If problems still persist, please make note of it in this bug report.