As has been stated earlier in the thread and evidenced by the language from Avis themselves when responding to the reservation, they want to check that the renter is entitled to use the company specific code and isn't attempting a fraudulent usage of a code.

Why do you think Avis is "doing anything" with the email address? Surely like any other business email user you've long given up any thoughts of privacy over that email address given the number of times you've given it out to others.