There are many projects that restrict US citizens from participating in the ICO in any form because they are trying to be in compliant with the SEC, which is not necessarily a bad thing.
it's actually the opposite. they block US investors because they don't want to be under the scrutiny of the SEC. there is a lot of uncertainty regarding ICO regulation in the US so it's better to just not deal with it. so if they block US citizens from investing, they are not under the jurisdiction of the SEC