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not that it applies to me as a Canadian citizen, though to clarify for any US citizens reading...

what is Rule 59?

would a trader still be held liable if the exchange sold to them, with their account registered in the US?

are the terms and agreements for the ICO that US citizens are not allowed to hold any of the EOS tokens, period - or rather that the company distributing the tokens are not allowed to sell to US citizens?

The SEC rule 501 (not 59) prohibits a company from selling an unregulated/unregistered security to public investors. But it doesn't prevent US public investors themselves to go and buy these securities through foreign exchanges (not sure about exchanges registered in the United States). So the terms and conditions are usually to protect the company (in this case EOS) from any litigation. But under certain exemptions these securities can be sold to 'Accredited' US investors.

Basically, what needs to be understood is that US investors are protected from any security fraud by having rules that prevent company's from selling unregulated securities.

Excellent clarification. 👌

Thanks!
btw do you have any tips to get popular in steemit? I hardly get any views on my posts :(

Unless I'm mistaken, the citizen wouldn't have violated anything as the onus and restriction is on the exchanges.
Crypto-startups not including US citizens in their ICOs is a precaution for any possible regulation that may eventuate - and don't forget many of these are truly global startups where, for example, something legal in Sweden may not be legal in the US - but I have never read anything that has made me feel that US citizens (and no, I'm not one) have anything to fear from the SEC. Yet.
As far as I understand it, as long as you're including any gains on your tax return you are obeying the law as it stands in 2017.