HDMI Org explained to TechRadar that there were a number of reasons for this. "The HDMI specification defines an HDMI cable as having only HDMI connectors on the ends. Anything else is not a licensed use of the specification and therefore, not allowed."
That quote from HDMI.org gives the impression that male HDMI - male DVI cables would also be out of spec. To say nothing of the scam HDMI-component cables that turn up on Amazon.com.
I'd never seen a male HDMI to male DisplayPort cable before hearing about this article. The adapters I've bought have all had female HDMI sockets.