Renaming not supported across conflicting directory - why?

I wonder what is point of checkign file/dir create mode like this?

Here it makes problems since some folders have different permissions than others but both are accessible/writable by user. So renaming is possible (as renaming according to unix permissions). Yet dovecot artificially prevents this.

    /* if we're renaming under another mailbox, require its permissions
       to be same as ours. */
    if (strchr(newname, mailbox_list_get_hierarchy_sep(newlist)) != NULL) 

{ struct mailbox_permissions old_perm, new_perm;

            mailbox_list_get_permissions(oldlist, oldname, &old_perm);
            mailbox_list_get_permissions(newlist, newname, &new_perm);

            if ((new_perm.file_create_mode != old_perm.file_create_mode ||
                 new_perm.dir_create_mode != old_perm.dir_create_mode ||
                 new_perm.file_create_gid != old_perm.file_create_gid)) {
                    mailbox_list_set_error(oldlist, 

MAIL_ERROR_NOTPOSSIBLE, "Renaming not supported across conflicting " "directory permissions"); return -1; } }

-- Arkadiusz Miśkiewicz, arekm / ( maven.pl | pld-linux.org )