You mention gmail which is a "mail" service, and whatsapp, which is a "messaging" service.
In all of (Western) history, those services have been considered one of the most private, like it or not. So changing that status (in the West) should require very public and very clear discussion, not just hand-waving about "child molesters".
Facebook: that is something totally different.
The fact that mail is handled by a private entity does not make it less mail than the official post. At all, and this has been understood (in the West) since, probably, the Roman times.
I am afraid to be difficult, but postage IS checked. As an example, try sending around drug and you'll get caught.
Government is proposing bills not have messages read by officers (something that not even inside Meta can be done due to EU's Electronic Privacy Directive) but to "scan them": have machineries check for violating content, pretty much as a parcel is put under xrays in the airport to check for bombs or drugs.
The law protects also the right of a company to protect itself against being blamed for misuses of its services, not only individuals' rights.
Snail mail is still scanned for illicit content. Also many times in western history letters were read and then possibly censored at processing facilities.
Back in those days, it was a human opening letter/package and inspecting the content. They had rules to respect the privacy of the sender/addressee, like not sharing the content with anyone else if the content is OK.
I'm not sure who considered these services to be one of the most private.
Also in those days, one could make use of a secure private mail service, bypassing the normal processing facilities and have grantee that the letter remained sealer the whole journey. To be even more secure, basic encryption could be used, or even invisible ink.
In all of (Western) history, those services have been considered one of the most private, like it or not. So changing that status (in the West) should require very public and very clear discussion, not just hand-waving about "child molesters".
Facebook: that is something totally different.
The fact that mail is handled by a private entity does not make it less mail than the official post. At all, and this has been understood (in the West) since, probably, the Roman times.