How would you use it otherwise? My backyard is private, but I share it with a few 3rd parties. That doesn't mean i intent to share my backyard with the entire world.
There is an element of trust with particular 3rd parties that is being violated. Why is that so hard to understand?
Unless you run your own mail server, someone knows your email address, so are you trying to define that as not private? Obviously, we are using fuzzy terms about private/public when there is a huge gradient of privacy.
The party has its roots in the skinhead/neo-nazi organizations and they have been trying to shake that image problem for quite some time now. To be fair, putting on suits has helped them.
95% of the population has absolutely no clue about how technology works and how it will be used against them, sooner or later.
Why not public? I guess it's fine if everyone knows it assuming a perfect spam filter (they aren't perfect btw) but I don't want it to be public knowledge what websites I use and what I say on those sites. Non-sinster example; I could be publicly discussing a sexual encounter and just not want the whole world to know (non-psuedonymously) that I did that.