[twinkletwinkle]

Tweets have the side effect of being public. Sometimes that becomes the main effect. Emails and the rest of your examples don't have that property.

[kbsletten]

You've clearly never had somebody forward your email on you. All digital communication is effectively public by virtue of fast, easy copying.

[what_ever]

There is a difference between public nature of tweet where anyone can go and read it vs public nature of email where you would need to forward it to everyone (or one of them would have to tweet it which is back to what GP said).

[vuln]

Transparency!