Nobody forces you to work for me. If you can't be stoic in face of retards without manners for (big) money, you can always collect welfare checks.
Very easy math. If I expect the employee to bring in... let's say 60.000€ in sales per year. If he causes a customer to drop who brings in more than that per year, I am putting my business at risk and with that also other people who work for me.
So, tell me again. What kind of business practice is it to insult customers? I would like to read up on that methodology.