For better or worse, the .US rules don't seem to stop anyone from registering them although an audit is possible and they are supposedly checking some domains randomly at registration. I have registered several since that time and have never seen this though. I am a US citizen, so this doesn't matter for me.
You are supposed to be a citizen OR have a business or organization presence in the USA. Hosting must be in the US or it's possetions. That is it in a nutshell but you should read the rules at the registry.