And I wouldn't expect a 2-3 year old to completely get the whole permanent marker thing.
I do expect a 22-33 year old professional to understand that you don't write on others' belongings. What would this teacher do if her students wrote on her belongings?
Let's take this to a different hypothetical... what if the teacher took the same sharpie and wrote a kid's name on the outside (not in the pocket, inside, or label) of the kid's sweater or hoodie? Would people still feel the same way?
That happened to us just last year when ds was in K. His teacher wrote his first and last name along the bottom of his jacket. (outside, on the back - if that makes sense - clearly visible if you were standing behind him!) He'd lost one on the playground the week before, so I could understand why she did it. However, his last name was already written on the tag inside! Why wouldn't she look there first?

Was I annoyed? Very much! Did I say anything to the teacher about it? Nope. I understood why she did it and didn't think it was worth causing silly stress for ds in the classroom.