It feels like Proton are being intentionally misleading in their statements. They know that most of their customers aren't familiar with how legal process actually works, so are happy to spread half-truths.
Under US law, a US law enforcement agency (LEA) typically has to apply for a subpoena or search warrant with a US court. The court is then responsible for deciding if the legal bar for search a request has been met, then either grants or denies it.
The problem is, if a company has no real US footprint (no US corporate entity, offices, servers, etc.), then a US court typically doesn't have the jurisdiction to compel the company to hand over customer data (except in some rare circumstances). Even if the court approved the warrant anyway, it wouldn't really be legally binding.
Which is why the Mutual Legal Assistance Treaty (MLAT) exists. MLAT enables law enforcement agencies in one company to send requests for information to law enforcement agencies in another. Switzerland has such a treaty with the US. This means that the FBI can request that Swiss authorities hand over a Swiss company's data on their behalf.
Any country requesting information held by a company in a foreign jurisdiction would typically do so via MLAT. Which means from Proton's perspective, the legal request would appear to originate from their local law enforcement, not the FBI. Which they clearly understand based on their Reddit post.
Saying "we don't respond to legal requests from anywhere other than Swiss authorities" seems very intentionally worded to give the impression that the company does not cooperate with foreign law enforcement. But since it'd be the Swiss authorities handling any such requests, they'd have to comply, since as they admitted, they have to comply with local laws.
There is, however, some useful (but more nuanced) information here:
Firstly, MLAT requests are handled by local law enforcement according to local law. So if there is a difference between the law of the sending and recipient country, that might mean the MLAT request is denied. That probably doesn't mean much, because if you're on the FBI's radar, the chances are you did something that is also massively illegal in Switzerland too.
Secondly, they are 100% correct in saying that no other service provider is going to do any better. They're all beholden to local laws, and the ones that think they're not tend to get their doors blown off by SWAT like CyberBunker did. The only exception is if the company resides in a country which does not cooperate with US law enforcement (which Proton does not).
But the part that's extremely disingenuous is that the "we only respond to requests from the Swiss authorities". That statement is likely intended to imply they don't cooperate with law enforcement in any other countries, which is simply not true. Switzerland has MLAT agreements with over 30 counties.
People really need to understand that no company is going to shield you from the FBI (or any reputable law enforcement agency). They'll use misleading statements to make it sounds like they don't cooperate with law enforcement, but they do. They have to.
