they doubled down on previous assertions that issuing or brokering tokens for crypto triggers money transmission (AML/KYC) law. after the SEC made this very explicit, we saw lots more ICOs prohibiting investment from the USA.
well most of them aren't even in US to want to comply with US laws. they come from Asian countries. and as you mentioned many of them even reject US citizens so asking for KYC becomes even shadier! besides we have already seen lots of KYC related documents that people submitted for the ICOs have been sold on the dark market.
don't get me wrong, i'm sure many ICO scams were carried out and many KYC documents sold on the darknet. i just think that is a symptom of a larger problem, which is legitimate fear of crossing authorities in the USA.
it doesn't matter if an ICO is set up in singapore or somewhere else. if the SEC and FINCEN determine you are illegally transmitting money on behalf of USA residents, they can shut you down and seize your domain/servers, freeze bank accounts, etc. that's what happened to btc-e and 1broker, all because they allowed people from the USA on their site. i know the SEC has shut down several ICOs too, though i don't remember specifics.
the first obvious step to prohibit USA investors is using geolocation IP banning. the next step is to implement KYC to ensure USA investors can't sneak in using VPN. whether the 2nd step is necessary, i'm not sure, but i'm very confident not all ICOs that did KYC were scamming.