The United States is usually a banned country. This is because the United States has laws protecting investment, which these projects do not meet and can be sued by the United States. This is bad for any project. Some other countries are forbidden because investors have short sight, they are willing to sell as soon as profit goes to the project dead.
This is not completely correct, the reason of why icos are not accepting investors from the United States is the very same reason many foreign banks and many other financial institutions do not want to accept investors from the United States and that is because those institutions do not want to deal with all the regulations that they will have to go through in order to accept an investor from the United States, it is simply not worth it for them.