The main reason why they cannot be purchased by people in the US is because they would be considered securities by the SEC and thus would fall under their tight jurisdiction.

The tokens are available for resale second hand on exchanges and they are a buy at your own risk kind of deal because the blockchain hasn't officially been launched yet. They cannot guarantee that the blockchain will in fact be born or when and if it will even be used but these tokens will be transferrable onto their network once it does come online, which it most definitely will.

I imagine it like building a really expensive yacht and then not taking it out because you didn't have a captain.