Este é um recurso de segurança, descrito na página do namespace do PID .
While processes may freely descend into child PID namespaces (e.g., using setns(2) with CLONE_NEWPID), they may not move in the other direction. That is to say, processes may not enter any ancestor namespaces (parent, grandparent, etc.). Changing PID namespaces is a one way operation.