I think I should only work harder for customers who treat me better.
Hm, why do you think that customers' actions should affect your work ethic?
Customers always want their crew to move faster and always want service with a smile.
Regardless of how customers are treating you, you should still work hard at your job.  Your employers are expecting you to treat every customer with respect.
How do you know that my boss doesn't want me to treat my customers this way. She said that she won't be here since she have many things on her plate.
Your boss doesn't want to lose customers because you didn't treat them well. We all have had customers we wish would go away, but if you value keeping your job, you will treat customers well.