Permitted or not, lots of packages do it. 32 on my system.
Just try doing this on your own system:
grep 'rm -rf' /var/lib/dpkg/info/* | grep /etc
A lot of packages put configuration files into directories that
belong to other packages. Maintainers of such packages should
make sure that the "host" package's postrm doesn't wipe out
their configuration files on purge.
--
Thomas