chroot's aren't namespaced, which is a big important difference. Being able to kill the entire container by killing it's init process (and keep it's processes out of the host namespace) is a huge deal.
Most of these implementations actually kill all processes in the namespace, not just init. First, there may not be an init (the root process could be the service, which is actually a best practice with respect to containers); and second, the signal may not be propagated to its children, especially if a non-trappable signal (e.g. SIGKILL) is sent.
