We can confirm these emails are legitimate, the links and headers all point back to blizzard.com servers. And indeed a blue over in the Customer Service forums has more or less confirmed that it's legit.
These kinds of promotions are not all that uncommon in the MMO world. Several companies that have MMOs of varying degrees of success have offered them in the past -- they're a nice way to get some old players back and make them feel like part of the community again.
We want to be sure we're clear: it appears these emails are not going to all unsubscribed accounts. If you have an unsubscribed account, don't count on getting one; right now it looks like it's just random chance whether or not you'll get some free game time.
Thanks to AishaLove for the help!