So what's probably going on is that she eats the mother in laws bento. Then some grocery stores will let staff take home unsold food on its expiration date, but Machi wants to be well liked by everyone all around, and has decided the best course of action is to throw away the unsold food (which has to be thrown out anyway due to expiration) and let her mother in law cook for her.
However, she still takes the unsold food because it was given to her. "They give me the unsold ones on the shelf before I leave." A refusal of a deed done in good faith, especially when it comes to food, makes the person refusing seem rude or too good for it. That's why it's hard to turn people down when they invite you to go eat or when they ask you to eat something for them, cause it feels like you're being rude if you turn 'em down.