Why when buying coins you need to check off 3 things, 1: agree to terms 2: confirm you are NOT a US citizen?

and 3: white paper.
i can not proceed without checking off the 2nd one. US citizens can not buy? I'm confused. Is it for legal reasons?
Due to more strict rules in USA, new ICOs have begun excluding US citizens.
One of the first to do it was
https://www.bitcoingrowthbot.io/. Some US Citizens where disappointed when the site rejected their registration. But later felt relieved when good evidence was brought up showing their creator Jordan Lindsey had record of being a scammer.
https://www.valforex.com/2017/09/aggressive-returns-review-a-scam-by-jordan-lindsey.html Whereas, Eigencoin is providing a way in. All US Citizens that have bought their coin have had to mark #2
