POSIX does not specify the init/rc script system, so it's not a factor here at all. A POSIX-compliant system could use Python scripts. macOS (which is UNIX 03 certified) uses launchd. A POSIX system has to ship the shell, not use it.

And FreeBSD isn't actually POSIX-certified anyway!

The real consideration here is simply that there are tons of existing rc scripts for BSDs, and switching them all would be a large task.