I've set my only printer to be the default printer via CUPS, which lpstat confirms
$ lpstat -p -d printer MFC-9840CDW is idle. enabled since Fri 03 Jun 2016 09:05:46 AM Online system default destination: MFC-9840CDW
Nevertheless, trying to print something via lpr does not work as it should:
$ lpr rn2016129.pdf lpr: Error - no default destination available.
(same is true for lp)
It is only when I explicitly set the printer name for lpr, things work:
$ lpr -P MFC-9840CDW rn2016129.pdf
The obvious question is: why?