Not sure where to put but: Is this fair?

I was at the library when I noticed someone had with an eraser written some stuff with it on a desk there. I had and eraser and made a little mark(By accident kinda not paying attention) and a librarian walked in and that moment and saw all the words, and she started getting all angry almost yelling at me to get a rag or something and clean it up. When I tried to explain what had happened she just interrupted and told me to wet some paper towels and wipe it up. So i did but then she started going on about how I need soap and how it's a government building and she could call the cops, ect. what would you do in this situation and was it really all that fair?