Currencies and international legalities question

I know this is a little offtopic but since there are a lot of CF developers on this subforum and this could benefit people using this theme, is anyone knowledgeable on the topic? Mainly interested in issues regarding U.S. based sites and accepting projects from abroad, accepting project funders from abroad, whether using a processor with just the dollar as currency makes things easier, and which countries need to be specifically excluded. I know some sites run US based projects only while accepting funders from across the globe while others only accept US posters and funders. What is the reasoning behind it? Thanks