Blizzard have changed the way Hearthstone's card pack system works in China, probably in response to Chinese laws that now require games companies to disclose the exact odds of receiving each class of item in any sort of loot box.
This includes Heathstone's card packs, which fall foul of China's gambling laws and are now being changed so that you can no longer buy them. HearthPwn spotted the change.
However, you can buy Arcane Dust, the in-game currency used to craft cards, and Blizzard will give you a card pack with every Arcane Dust you purchase. For now, that seems like it would satisfying the legal requirements the company face regarding disclosure of odds on their card backs. However, it's clearly an exploitable loophole, so perhaps this isn't the last we'll hear on the matter.
You'll recieve an equal number of card packs as Arcane Dust purchased. The price isn't yet clear, but it would be unsurprising if Arcane Dust sold for the same price that card backs sell for now. For reference, even the most basic cards in Hearthstone often require 400 Arcane Dust to craft, so a single mote of the dust is functionally worthless.
For now, this is only available in Chinese servers, and it seems likely that it'll remain limited to the country.