PARCC, Social Media, AFT, and "Privacy"

So I'm taking a break from this (rather slow) description of my transition from teaching to the "other side" to ask this question: Why do people still believe that they have an expectation of privacy on social media???!!!! Perhaps like me you recently received this email from the AFT:

Warning: Rant headed right for you.

Now let me say straight up I am no big fan of Pearson or the PARCC, defending them is not my objective here. However, it makes complete sense to me that they would contract employees specifically to monitor social media for public posts regarding the test. This email (which conveniently provides no link to the original article by the way, including that hyperlink you see in the reference - it just leads to AFT's petition) defends the student by saying it "was nothing more than a tweet." It was upon reading that phrase that my brain began to bleed - TWITTER IS PUBLIC! 

Why do we keep waffling back and forth between telling students they need to careful about what they post on social media yet constantly defend their "right" to privacy? If you put something out there for everyone to read, then you have no control over who reads it. Period. You're "worried about a generation of kids growing up without a moment of privacy"? Teach them that social media is NOT PRIVATE! 

Really, how hard is this to understand, people? So no, AFT, I will not be signing this petition. Doing so would be agreeing that students don't need to think before they post, that they have a right to privacy in a completely public environment, that they need not be at all concerned about potential consequences of their actions. It would contradict everything I have tried to teach them about their online safety, which will just make them more vulnerable in the future.

And I realize that people will disagree with me on this, perhaps vehemently. I realize I may even get some angry responses. Why do I realize this? Because I understand that posting this in a public forum means that it is not private.