That was my thought too. Nonetheless, that's a technical misfeature (they assumed the walled garden provided all the authentication they needed, and it doesn't), and technical misfeatures happen all the time. Now they have to decide whether to implement that feature or not, and that's a business decision: they know what the (legitimate) sales numbers are already. If those aren't enough to justify the cost, then it's simply not worth it and they should just shut it down.
Or, more cynically, they know they can get a bunch of press (and thus, more sales) by "shutting down" the game temporarily while they implement the authentication layer.