I spent a bit of time tonight speaking with the helpful people over at Blizzard and learned a couple things about Pre-ordering Legacy of the Void; if you were hoping to get early access to the Whispers of Oblivion prologue, don't bother with a physical copy of the game, even the Collector's Edition. This bonus applies only to those who have pre-purchased the title through Blizzard digitally.
There is a benefit to obtaining the physical copy through "select retailers": Beta Access. If you are looking to join the beta at least, that's a good way to get into it. Hopefully Blizzard will look into altering this lack of prologue, but I wouldn't get your hopes up. Patience is a virtue for those who want a physical copy of the game.