On the EOS page we can read:
NO U.S. OR CHINESE PURCHASERS - EOS TOKENS ARE NOT BEING OFFERED OR DISTRIBUTED TO U.S. PERSONS OR CHINESE PERSONS.
Why these groups of people are excluded from EOS tokens?
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They need to protect themselves from the SEC which is in the middle of determining a classification for crypto currencies and ICOs.
They have no means of actually preventing you from purchasing, owning or otherwise transacting in EOS on the ICO or from a 3rd party exchange.
Further, some have expressed concerns that users from these countries who purchased EOS despite the USA/CHN ban will not receive their coins during the June launch. This is incorrect, there's no means in place of detecting your location before handing out the tokens. So long as you register your tokens properly you will receive the appropriate amount of EOS Coins after June 2nd.
Their are various legal reasons why it is more difficult to accept direct investment in direct ICO purchases for EOS. Many projects choose to exclude US or Chinese citizens because the law under which the government will enforce rules is not fully codified and therefore creates uncertainty, or the laws and regulations are more stringent than other countries.
You can purchase EOS tokens from crpytocurrency exchanges such as Binance or many others.
Right now there is a legal controversy in the US, to decide whether crypto currencies are considered a security or not. Usually a security offers some kind of dividend (in this case possibly Airdrops). If EOS.IO is considered a security, it will be banned or penalized in the US. I think EOS.IO wanted to mitigate any legal ramifications by excluding the US territory. Therefore it won't be scrutinized as a security if it's not allowed to have it's ICO open to US residents.