It is completely legal. The law does not require a customer or client to be respectful to vendors, contractors, or workers--they are allowed to treat you as awful as they like. The law also does not require them to keep employing you: just as you could refuse to go back to a hair stylist, accountant, lawyer, or doctor who you did not like, the customer can refuse to keep using you. Yes, this very unfair--but it is perfectly legal, unfortunately.