The man pages don't mention these config files in /usr, i.e. you get weird effects due to some undocumented config files.
Its unexpected (esp. since /etc/lightdm/ can be found in the usual directory) and it doesn't follow the policy. The policy is very strict in this case: Any configuration files created or used by your package must reside in /etc. If there are several, consider creating a subdirectory of /etc named after your package. If your package creates or uses configuration files outside of /etc, and it is not feasible to modify the package to use /etc directly, put the files in /etc and create symbolic links to those files from the location that the package requires. Is this unreasonable or invalid in this case? Regards Harri