Yes! That sucks.
From what I understand, that's their way to do e2e encryption between proton emails. They have this bridge thing, which unfortunately you can't use on free plans.
Proton Mail Bridge is a desktop application that runs in the background, encrypting and decrypting messages as they enter and leave your computer. It lets you add your Proton Mail account to your favorite email client via IMAP/SMTP by creating a local email server on your computer.
I agree that this is a problem, and that it is worse than Google.
But for me personally, putting it all on a scale I still think it is better than having your emails mined for ads targeting.