These accounts were reportedly associated with third-party programs that automated gameplay, in other words they were botting. The “good news” is that it does not appear to be a permanent ban, but rather a ban that is expected to last until 2015, so if you got caught then we guess you’ll be out of action for the next few months, and hopefully come 2015 you will think twice before botting again.
However as Blizzard stated on their blog, future bans are expected to be permanent and without warning, so if you got caught now, we guess you can count yourself lucky. “We’re committed to creating a fun and rewarding environment for our players, and we will continue to closely monitor activities within Hearthstone and take appropriate action against cheating in any form, as outlined in our Terms of Use. From this point on, accounts found to be cheating will be permanently closed without warning.”