Proton is end-to-end encrypted - they don’t have the keys themselves. With TLS, encryption is between you and the server, but the information can be decrypted on the server side.

At least that’s my understanding of it. If you want Proton’s own words, they wrote an explanation on their website. :)

I presume ProtonMail markets this as a security feature? It guarantees your provider cannot sniff your email content and can’t be compelled to disclose user emails to law enforcement.