It's rude to use something someone is already using.
Sally is shopping for a small item in Costco without a cart.
Sally wants to buy a toothpaste packet.
After picking up toothpaste, Sally asks nearby customer if she can use their cart to load the discounted microwave.
Besides getting a toothpaste, Sally sees a huge discount on a microwave, picks it, and puts it in a nearby cart.
Sally gets permission to use the cart and feels glad to buy the microwave at such a low price.
A customer becomes upset with Sally as the cart belongs to them.