How so? I'm entirely willing to listen to your opinions, but please back your statements with something lest you be taken for a troll.
Because it is an alpha version. It is an incomplete game.
I shall bring you an automobile and charge you full price, but this vehicle shall be missing its outer paneling, have no air conditioning, and the windshield wipers work only when it is not raining.
This vehicle will still run, and you can drive it around, so you should pay full price.
It's more like "I'm going to sell you an automobile for half price. It works, but it isn't complete (missing some nonessential stuff). However, over the next year or two, I will add parts to it until it is a complete automobile". And it's not even full price (I would gladly spend $30 on Minecraft anyway).
It is a bit silly, but you're paying half price to get the same thing later. It's still a ton of fun now, I've put more hours into it than I have with a lot of other games I've payed full price for. The point is that you're not paying for incomplete software, you're paying for what is essentially a full game now, with some extra features that will be added later. If Notch said that Minecraft was a full game that was being patched over time, then this probably come up. Classic was the REAL "alpha" version of Minecraft. It is essentially nothing more than a basis for the real game. Survival test was late alpha/early beta, and indev/infdev were late beta. For all intents and purposes, Minecraft is a complete game. Compare it to an MMORPG, but getting all the "expansions" for free.