Credit to Amazon reviewer Sam_One, who noticed "something weird" going on back in May:
Something weird seems to be going on with Warner Bros. pre-orders. Loads of them are not available anymore, and all of them were removed pretty much at the same time (I had several of them in my "Saved for Later" list. This includes most releases of 300: Rise of an Empire, The Nutty Professor Box Set, The Man who knew too little, Looney Tunes Platinum Collection Vol. 3 etc. etc. All of them Warner releases, some of them pretty big.