I seriously doubt that it's sold out. This happens frequently on Amazon. I don't know why, but at some point after beng announced for preorder, titles are then made unavailable for ordering. Then, usually, about ten days prior to the actual release date, the title becomes available again. I strongly suspect that on, or around, June 1st the BD and the DVD will again be set up for preorders.
I suspect that it has something to do with HBO also selling the title on their own website.
Bad news...The allocation for Amazon has indeed been sold out via preorders. I was told this directly by a supervisor in customer service. I just ordered a copy from Walmart for $39.95 with free shipping. It will arrive 3 to 6 days after the release date, but it will arrive. I had to do the same thing for the Falling Skies: Season 3 Blu-Ray...which was only $24.95. It's not huge savings, but it is small comfort.
WB distributes some of the HBO shows and movies and WB is currently in contract negotiations with Amazon. As soon as they are finalized, you should see pre-orders available again (provided the contract(s) are finalized before the release date).
Call Amazon customer service within the next seven days and you can get that ten bucks back! Same with any purchase from Amazon (as in sold by Amazon not a third party or just shipped by Amazon). I saw that it dropped by ten today and was annoyed too so I called because hey ya never know... and that's how I found out about this policy.