Comment 4 for bug 1585087

Got it -- bamfdaemon also installs /usr/share/dbus-1/services/org.ayatana.bamf.service which calls bamfdaemon-dbus-runner which does

if [ -n "$UPSTART_SESSION" ]; then
  /sbin/initctl start bamfdaemon
else
  /usr/lib/x86_64-linux-gnu/bamf/bamfdaemon
fi

That explains why its upstart job is D-Bus activated and the "start on" condition is redundant (and disabling it doesn't work).

Adding "manual" to e. g. im-config.override does work as expected.