Read the passage, and answer the question that follows.

Amber entered the small grocery store. She looked at the list that her mom had sent with her. She knew that she only had enough money to buy the milk, bread, and eggs that were on the list. Quickly, she grabbed the items and paid for them.

Eying the candy on her way out, she picked up a pack of gum, looked around and saw that no one was watching her. It had been weeks since she had chewed gum, and she thought that it would be okay. She had walked a long way to the store and felt that she deserved a little treat.

Amber walked through the double doors staring at the bright green package of chewing gum in her hand. She immediately turned back around, walked back inside, and returned the gum to its spot on the display stand. The cashier smiled and nodded, but did not say a word.

Why did Amber put the gum back on the shelf?