Debian doesn't follow the Puppet post-install instructions because you have to also manually modify /etc/default/puppet to allow the service to start. That's non-obvious unless you're experienced with running services on Debian.
The default script was added because debhelper started services on package install, while we require some configuration before the service can start. With the puppet-agent AIO package, services no longer start on install; they have to be manually started (as instructed in post-install), which means we can remove /etc/default/puppet and the extra install step that adds.
Risk Assessment: Low
Testing Layer: Spec