That would be because there are none. The US considers most ICOs securities, so companies will have to comply with US regulations if they want to cater to US customers. Some companies find this way too troublesome, so they simply implement an in-house rule that they don't deal with US citizens. I don't think using a VPN to circumvent this limitation is fraud (in a criminal sense at least) either, but they may choose not to honor your holdings and there's nothing you can do about it.