You could argue about many things bundled with systemd, but since containers are just souped-up processes, this is actually an use case that make sense for an init system.
I disagree. I would prefer to have a model where the supervisor needs no knowledge of the LSM profile, namespaces, seccomp profile, or cgroups applied to a service. Additionaly, I would prefer a non-init service supervisor.
I see them as souped-down virtual machines or the new vServers but with a decent interface. vServers (a bunch of hacks to separate users as if running different Linux instances) were pretty awful.
Access control is at best problematic.
Upgrading the daemon without losing state is tricky.
Requiring daemon access to build images is insane.
Building this functionality into something like systemd would be more robust but it's way harder to sell as a product.