You are not subjected to US law. However verisign have complete control over .com namespace, and they are subject to US law and US courts. Verisign will take your .com off you if the US counts tell them to.
The exact same thing happened to Arrington in the UK with regards to a frivolous libel suit he chose not to defend. He couldn't visit the UK or risk being arrested.
National laws differ. We each make our own. If you are found guilty of a US law, either (a) don't do that, or (b) don't come to the US.
This is no different than any other country on the planet, the US is just bigger, so their laws may impact you more than Iran's, but no-one is forcing you to visit or do business in the US or Iran.