That's a great point. No need for moderation through postal mail because it's all 1-to-1 communication. OTOH, all broadcast media is heavily moderated today.
Lol postal mail has a long history of moderation. A lot of first amendment case law with respect to obscenity and fraud comes from postal cases.
Do you have some examples of what you're talking about?
The Comstock Laws [1] are probably the most famous US example, which made it illegal to use the USPS to send any obscene materials, including personal letters with sexual content.