Postcard Etiquette. Is there such a thing?

I have always had a love of post cards. Whenever I travel, or have friends that travel, I either pick some up or have some sent to me. This was why when I found the website Postcrossing I was ecstatic!
If you have never heard of Postcrossing it is a site that gives you addresses of people around the world who would like postcards and gives out your address to people from other countries in exchange.
Its great!
Just today I was given addresses of people in China, Finland, the UK, Russia and Brazil. So I fill out my postcards and head to the post office for postage.
When I walked into the lobby, I found, for the first time I believe in my entire 28 years, not only was there no line, but I was the only customer there.
Instant service ensued and soon I was putting 98 cent stamps on my post cards. Since there was no one there I stayed at the counter to put the remaining stamps on and the clerk said she would help me out by putting the "Airmail" stamp on them. As I finish with my portion I look over at her and what is she doing? She is reading what I wrote on the card..
Now, these are very impersonal cards.
Typically all I end up saying is "Hello! My name is Rebecca and I am from the city of Portland, OR. This card shows.... [whatever local feature I decided to share with this person]... blah blah blah... Hope you have a great day!" but that is beside the point!
What makes people think that just because it is not in a sealed envelope they have a right to see what it says?
What if it was in an envelope with the flap open? Would they take it out and read it? I think not. In fact, reading other people's mail is against the law in some states. Sure, there is an expectation of privacy with letters, but if the card is not addressed to you do you really have a right to read it?
I think it would be interesting to send a card somewhere with a tally section stating "if you have read this mark here" to see how many people actually read a card from point A to point B..
I have a feeling that the results will astound me.
Would anyone like to be my point B?
Email me :)