But this is not about alternatives to WhatsApp, imo it's weird that this even happens/is possible.
It’s not. This will happen with any chat app that is locked to a single service. Stop using those.
If you have a secret to keep don’t share it with a phone.
Neither WhatsApp nor Facebook can read your messages or hear your calls with your friends, family, and co-workers on WhatsApp. Whatever you share, it stays between you. That’s because your personal messages are protected by end-to-end encryption. We will never weaken this security and we clearly label each chat so you know our commitment.