Facebook Messenger does actually offer an end-to-end encrypted service, though it's not the default setting.

They're exploiting the fact people don't know how it works. They see "https", they think cops can't see anything

Except it is encrypted, and pretty secure. That's not really related to the issue. Facebook complied with a subpoena as they are legally required to do so. Signal would have to do the same. The only difference there is that Signal doesn't retain decryption keys for your data so subpoenaing them would be pretty pointless except to prove that some conversation happened.