I don't know much about Proton Mail but presumably they want that so they can actually provide you with a more complete service, other than just being a mail gateway?
I would assume that any technically sophisticated users who just want an SMTP/IMAP server would never let their keys leave their control, but there might be other users for whom a "middle layer" service which has their keys is good enough. (I guess this is especially evident in cryptoassets where people seem to cheerfully let third parties manage their tokens, so it's not really surprising to me that there are a bunch of people willing to do it with their PGP keys for email purposes.)
I guess there's an argument about whether or not they're being responsible in providing such an option at all, which is fair enough.
I would assume that any technically sophisticated users who just want an SMTP/IMAP server would never let their keys leave their control, but there might be other users for whom a "middle layer" service which has their keys is good enough. (I guess this is especially evident in cryptoassets where people seem to cheerfully let third parties manage their tokens, so it's not really surprising to me that there are a bunch of people willing to do it with their PGP keys for email purposes.)
I guess there's an argument about whether or not they're being responsible in providing such an option at all, which is fair enough.