HDMI Org, licensor of the HDMI standard, has demanded that Mini DisplayPort to HDMI cables be taken off the market because they are "unlicensed." According to TechRadar, the cables fall afoul of HDMI licensing standards which state that any cable with a male HDMI connector at one end must also have a male HDMI connector at the other end. Anything else doesn't meet the standards and therefore cannot be sold.
This doesn't affect the sale of cable adapters, like the Mini DisplayPort to HDMI adapter sold on Apple's site, because said adapters have a female HDMI connector that still requires a separate, additional HDMI cable to plug into the display. Those types of adapters are okay under the HDMI standards, but a cable with one type of male connector on one end and an HDMI connector on the other apparently isn't. Picky, picky.
Those of you hoping to bypass the aggregated cost of a dongle adapter plus an HDMI cable in favor of a one-stop solution for hooking a newer Mac into an HDMI display had better act fast; such cables are still available on Apple's online store for now, but MacRumors points out they've already been withdrawn from Monoprice.