They’re not doing like proton and close basic stuff like IMAP and SMTP as a way to force you on the official apps
The reason Proton cannot do IMAP/SMTP is that they cannot read your emails which is required for both. That’s a feature, not a bug.
PM works with any app as long as the app implements their custom protocol for which there are at least two FOSS implementations as a reference.
proton is a “fake” open source that is mostly used for marketing: they opened only the UI, which communicates with a proprietary protocol to a proprietary server - useless
While I’d also prefer their back-end to be OSS, it’s not nearly as critical as the clients.
As a user, it doesn’t make a difference. I’m paying for an opaque service either way.
All the interesting stuff (E2EE, zero access storage) happen in the clients anyways. The BE is fairly uninteresting; it’s a mail server + zero-access encryption + Proton account handling. If you really wanted to build a mail service similar to Proton, you could build that yourself and probably would have to anyways.