Protonmail is not a person.
If the allegation is true, it looks like Protonmail decided to munge e-mails without anyone opting into it.
Spontaneously doing anything unusual with e-mails is a bad idea, until it becomes a standard behavior that everyone else does.
How about a prompt?
Your recipient foo@example.com has registered a public key with openpgp.org.
Would you like to encrypt this mail with they public key?
If you know that the recipient is prepared for encrypted e-mails, say Yes.
If unsure, choose "No".
[ Yes ] [ No ] [ ] Don't ask again for this recipient