That's interesting. I am on a sid system, and I haven't noticed systemd on my system. I have the support libraries:
$ dpkg -l | grep systemd
ii libsystemd-daemon0:amd64 204-5 amd64 systemd utility library
ii libsystemd-login0:amd64 204-5 amd64 systemd login utility library
I also checked a testing box, and it has the same libs.
I also, just out of curiosity, checked apticron on both boxes, but neither of them has systemd on the to be installed list. Maybe a dependency on something previously installed?