Indeed, it's not exactly as if Minecraft is ridiculously expensive either. It costs about half of what a new game costs these days, and on top of that you get 100s of hours of gameplay out of Minecraft.
While it probably doesn't make a difference anyway, as this guy was going to pirate regardless of whether Notch told him to or not, it's still the principle that counts.
If you don't have the money to purchase a product, be it digital or physical, you don't get the product. It might sound like I'm a dick, but why should Notch condone him pirating it simply because he's poor, but I have to buy it? We all work for what we earn, so being poor isn't a valid excuse to get freebies.
If I were Notch, I simply wouldn't have responded to the tweet.