My guess is they MAY actually verify email addresses for the gym membership before sending that $15 credit.
"After you successfully sign up for the Audible Trial,
and your account is verified, you will receive an email notifying you that you have received a $15 Amazon promotional credit in your Amazon account. "
Offer available while supplies last. Valid for U.S. Customers Only. Offer not available to existing audible customers. You will receive a 2-month Audible free trial that includes 2 credits as part of the standard Audible Monthly Gold Membership Plan. At the end of the free trial, your membership will continue until cancelled at the then current full price (currently $14.95 per month) with your designated credit card or another available card on file. Cancel anytime by visiting the Account Details page. After you successfully sign up for the Audible Trial, and your account is verified, you will receive an email notifying you that you have received a $15 Amazon promotional credit in your Amazon account. The Amazon credit will expire 90-days after it is deposited into your account. The Amazon promotional credit does not apply to digital content. The Amazon promotional credit only applies to products sold by Amazon.com (look for "sold by Amazon.com" on the product detail page.) Products sold by third-party sellers or other Amazon entities will not qualify for purchase by the Amazon credit, even if "fulfilled by Amazon.com" or "Prime Eligible". Shipping charges and taxes may apply to the full value of discounted items in your order. If any of the products related to this offer are returned, your refund will equal the amount you paid for the product, subject to applicable refund policies. Unless an Amazon Gift Card is the stated benefit of the promotion, promotional codes or credits (including those placed directly in accounts) may not be redeemed for Amazon Gift Cards. Offer is limited to one per customer and account, may not be combined with other offers, is non-transferable and may not be resold. Audible and Amazon reserve the right to modify or cancel the offer at any time. If you violate any of these terms, this offer will be invalid.