Longer answer:

Like most things in life, it all depends upon how you look at it: Gamestop and Stardock mean different things when they say "Pre-order".

Gamestop is offering you a pre-order in the traditional sense.  You pre-order the game, the game comes out, is shipped to Gamestop who then hands you a box.  At any time in this process you can decide you no longer want a pre-order, Gamestop gives you a full (or partial depending upon policy) refund, and it's as if you never pre-ordered.

Stardock's Impulse is offering a chance to buy into the game development process.  You buy access to the current build.  You play/test the various builds of the game, give feedback, and generally act as another set of eyes, ears, and mind in the group as the game continues to develop.  When the pre-launch cycle is finished, the game is released and you get the nice warm fuzzy feeling of knowing you helped make it all possible.  As a bonus, you also get the game for free.  This is not a demo, and there is no guarantee of fun.  There will be no refund possible since you are putting down money to be part of the development process, not to buy the game itself.