In other words, the law does not say that ProtonMail is exempt for having to provide lawful interception. They might still have to do so, based on some other law.
Further, it's quite conceivable that unlawful means (such as blackmail, threats, or bribery) could be used to coerce ProtonMail. That's not to mention perfectly lawful means of enticing them -- like appealing to their patriotism, willingness to help in a critical investigation, or demonstrating some credible threat.
Not all actions of the US government have survived legal review and some may argue the latest administration is more prone to such accidents.
Heck, the EU and US are already not on good terms on the subject of privacy/intelligence gathering, so i doubt this would be done so easily. Especially with the current US administration.
> "Nearly every country in the world has laws governing lawful interception of electronic communications. In Switzerland, these regulations are set out in the Swiss Federal Act on the Surveillance of Postal and Telecommunications Traffic (SPTT) last revised in 2012. In the SPTT, the obligation to provide the technical means for lawful interception is imposed only on Internet access providers, so ProtonMail, as a mere Internet application provider, is completely exempt from the SPTT’s scope of application. This means that under Swiss law, ProtonMail cannot be compelled to backdoor our secure email system."
https://protonmail.com/blog/switzerland/