I see that the token sale is a security token offering, hold on an exchange (first time I guess). But even if your sale is regulated according to SEC rules, you cannot allow US-citizens to participate? Isn't the SEC an US agency?
United States is an highly regulated environment. It's the most legal incline country you ever know. SEC not allowing US investors confirms the facts. US participants however may have the opportunity of purchasing it on the exchange when it starts trading. So it's not really a loss for US citizens.
I have not seen any ICO, IEO, or STO which allows US investors to participate in tokens sale. This tells that, additional approvals are needed and project might need to spend more money and time for doing that. However I believe US people can buy tokens after it's gets listed?
Yes. USA can buy tokens from only exchange that are fully in compliance with SEC rules and standards. I think it's a great way to help Americans not to loose their money.