I was looking at the page and it said that the source code was available, so I was just worried because can't people like take the source code of of the game and make a stolen version?

They could, but the game is free so there's no reason for anyone to download a copied version if someone did.

It's intentional. It's not only free as in free beer, but free as in freedom; except a few explicitly specified items, it's licensed under the MIT licence. Ergo, you can do (almost) anything you want with it