It's an early access game. Not finished but playable. I see it as a sort of payed beta testing. Kerbal Space Program Blog, Expansions, DLC and the future of KSP
So given that this was a point of confusion, and that we believe that no matter what, a promise is a promise, we are including Expansions in what you can expect to get for free if you have already bought the game. Also, for those considering purchasing the game, we will maintain this promise for all purchases made until the end of this month (April, 2013).
At a 50% price discount you get early access to the game, the FULL game when completed, and whatever updates / expansions that come afterward for free. Of course the offer is already over for those who haven't purchased it yet.
I think those who bought it early in development got a pretty good deal.