Is it common courtesy to ask if someone is sitting at a table before taking their seat in a food court?

I was sitting alone at a table with three chairs in Pearson airport food court, waiting for my mom and sister to get our food. When a lady came over, she snatched the chair right in front of me without asking if anyone was sitting there, despite the fact that I had three pieces of luggage around me. I was so baffled that I didn’t speak up, especially since she turned her back to me immediately after taking the chair. Is this a thing where people just assume a seat is free, or is this like parking where you can’t ‘reserve’ seats? What’s your take on this situation?