Why must PayPal button be used when balance is $0

Hi, I am going to be promoting a coupon code to join my community for free. I want the visitor to see the cost of a regular membership and be able to enter the coupon code, which will reduce the cost to $0 to join. I'm just wondering why the person still needs to click the PayPal button to proceed. It seems unintuitive. They don't need to pay anything. I think it will be confusing and discouraging. Any thoughts, advice, ideas?? Thanks so much for your help!!