I finally understood what one of my lecturer had said. The sole and main purpose. For imposing the $500 fine to those who are drinking and eating in the bus. It's not for making the bus dirty. It's for making other passengers who are hungry even more hungry.
Wah...... damn BASKET-ball lar! I after dragonboat never eat lunch till 4pm. Then when i was taking the bus got 1 auntie behind me eating Macdonald's. Whoa the alluring smell of the french fries and burger! Make me damn hungry lorrr. Wah then my stomach 不高兴 use gastric juice burn me. The bus company should really fine those people! :D