Is my work wrong, or am I?

Okay, here is my situation. I have worked at my current job for the past two years, its one of our local grocery stores (that will remain nameless). I'm also a college student, and I know this is just a silly little job to earn some extra cash, but this just really upsets me. Anyways, like I was saying, I've been here for two years, and have never been in any trouble until now. Today, I was bagging for the cashier at one of the registers next to mine. We talked to the customer, who seemed happy, I bagged up all of her groceries -everything was pretty normal. We get to one of her final items, and it happens to an alcoholic drink (the fruity ones in packets that you freeze into a slushy), and as the cashier passed it along to me, she casually asked me if I had ever tried them. I simply said "I don't like them", bagged them, placed them into the lady's cart and went on. I said nothing else. Well, apparently the lady didn't like me voicing my opinion about it, and went on Facebook (or twitter) and posted about how she didn't appreciate one of (insert my stores name)'s employees saying how "nasty" something she bought was. And of course since my upper level boss has nothing better to do than play on Facebook all day long, he saw the message and called me to his office. Again, he has nothing better to do than look on the video cameras and see exactly which employee the lady was talking about. After a short discussion, he decides to write me up, saying if it happens again, I'm fired. So, recap.. I'm in trouble because I don't like a product and some crazy lady with nothing better to do puts it up on Facebook. If I talk to the upper manager tomorrow, and if he doesn't retract the demerit, do you think I have a case with the corporate office?