The LA Token whitepaper mentions that US residents can participate because the LAT token is not considered as a security with the exception that the US residents cannot use some of the other features like the asset backed tokens on their platform. So US residents participated. Now there's an issue where this is not the case and that the LAT token may be considered as a security which can cause problems with the SEC regulations. That's why for most ICOs US residents cannot participate in ICOs. I think they also stopped allowing US residents to participate for the time being.
Someone feel free to correct me if my understanding wasn't correct.