It depends on jurisdiction. For example the UK has the infamous gag orders that are even harder to fight in court (successfully) than their US counterparts.

Sure, ProtonMail and its current operators could opt to stop operating in such jurisdictions, but usually it's too late for that when you get the [secret] court order, because if you refuse the operators personally are quickly found in contempt of court (or whatever other bad legal circumstance), and eventually that can lead to similar InterPol/EuroPol (other MLAT) warrants.