That's a bug IMHO which I reported at http://lists.gnu.org/archive/html/bug-bash/2015-02/msg00052....
I've collated other mishandling of closed pipes at: http://www.pixelbeat.org/programming/sigpipe_handling.html
I've collated other mishandling of closed pipes at: http://www.pixelbeat.org/programming/sigpipe_handling.html