@sterven
I think it's because feeding each other is something couples do. Komura is not yet (officially) her boyfriend, and he seems like the type of straight-arrow who would introduce himself to her family and possibly ask permission to date her. He wouldn't want her parents to disapprove of him, and he would want to show he has pure intentions toward their daughter. Like, that he's not just stringing Mie along to take advantage of her, or cheat on her, or break her heart.
Plus, Komura and Mie doing something that couples do in public might start rumors. Maybe neither of them thought of that, and/or Mie just doesn't care.