The HDMI Organisation said a DisplayPort/HDMI cable isn't listed in its specifications, so it can't be properly tested, according to a report on PCMag.
"The HDMI specification defines an HDMI cable as having only HDMI connectors on the ends," Charlene Wan, director of marketing for HDMI, told the site. "Anything else is not a licensed use of the specification and therefore not allowed."
"It is true that companies that make and sell cables that have a [DisplayPort] male connector on one end and an HDMI male connector on the other end have been told that this product is unlicensed and cannot be sold," she added.
The group has allowed cables that have a DisplayPort connector on one end and a female HDMI connector on the other, however. That would still require the purchase of an official, tested HDMI cable.
"It is illegal to make and sell non-compliant products," Wan added. "HDMI has the right to demand all retailers and channels to remove any and all non-complaint products from the shelves but [hasn't] done so."
Mini DisplayPort is featured on a wide-range of PCs, including Apple's MacBook Pro line and Lenovo's ThinkPad X1. Apple's online store offers a Kanex mini DisplayPort to HDMI adaptor, and they're widely available elsewhere.
The HDMI Organisation has yet to get back to us on what action it plans to take.