ProtonMail encrypts the contents of messages, but not the addressee information.
Obviously they can't encrypt the address of the sender or the recipient, otherwise the message would be undeliverable.
So, even though you can send an encrypted email to, say, a gmail user, Gmail and the U.S. government will still be able to see clearly that that person received a message from you.
(Edit: what I mean by "can't encrypt" is that at some point ProtonMail has to be able to read the metadata in order to know where to deliver the message. Certainly ProtonMail can encrypt the metadata to itself when the message is sent, so that it travels over the wire back to their servers in encrypted form. But if the message is to a non-ProtonMail receipient, then they end up permanently unencrypting some metadata simply in order to be able to send it on to the recipient at Gmail or Hotmail or w/e. The point is that, even in the best-case scenario where both parties are PM users, PM has to retain info about the messages between them in order for the service to function, and this important metadata would be included in the "extremely limited user information" that a Swiss court order could conceivably compel them to release.)