It should be noted that merely being accessible in the EU doesn't automatically make the GDPR apply to a site, although the line is not sharply delineated.
> What happens if a site operator in Iowa just ignores it.
Other European businesses, will be banned from doing business with them. For facebook they can stop them buying advertising space, for others they might issue orders to block payments to them. Some small players in Iowa may still get away from things but no one notable will.
Okay, they might get a letter or two that they place in the circular file.