Isn't that one of the pros of Protonmail? All the data is encrypted and decrypted on the client. There is no way to have mail apps access the data without a piece of software that handles the encryption.
In other words ProtonMail is not actually end-to-end encrypted which makes it snake oil.
It’s interesting that this point has to be explained on HN time and time again, as HN’s audience should understand end to end encryption.
I'd love to have a privacy-respecting mail hoster, but the ProtonMail bridge program is too unattractive to make the switch.
(Which is not to say that IMAP itself isn't a mess, but MUAs have gotten it to work, and we'd like for whatever other protocol we move to instead to also be an open standard.)