As a gentoo user, this postinstall behavior on debian/fedora/etc was rather shocking to me when I first encountered it. With gentoo portage, either when initially installing a package or when upgrading it, nothing under /var/lib is created or modified, and the process itself is never automagically started or stopped. The postinstall behavior actually feels like a regression toward windows-style installer.

The cassandra debian package is a good example. If I let the postinstall script runs, then it will use the default config to start a single-node cassandra cluster, creating a bunch of stuffs under /var/lib/cassandra, which then need to be removed first if I want to build a proper image from the disk. Doing the removal as part of the automation feels too risky to me, as it may only take one typo to accidentally delete /var/lib/cassandra in production. Instead, I usually just put a policy-rc.d script to disable all postinstall scripts from running.