I had the actual box set of SC2 Heart of the Swarm and even then I had to have the original installed before it let me do anything. So it forced me to buy a $10 digital copy and install it first. It is a expansion and not a stand alone game, just like Cata, MoP, etc.
"Even when you are winning soundly, you must always give your opponent a way to retreat with honor. If you don't, then he has no reason to surrender. He will fight until the bitter end, and you will pay a larger price for victory."