EU companies can do business with US companies as long as they're following the law. Thousands of companies and websites have no problem doing so.
They were warned because they were transferring email addresses to a company that falls under US Surveillance law and there weren't any relevant legal protections in place... and the company facing the fine never did any due diligence to check.
If you run a business and don't consult a lawyer about privacy laws you're taking a big risk... many countries have laws that you could easily run afoul, even other states in the US.