I believe I marked it for "all payments," but I will double check and let you know.
UPDATE...reply from Squarespace support:
At the moment, card details are required for all recurring subscriptions or memberships, even though technically the total will be zero, even for renewals. We don't currently offer another option, but I'd be happy to suggest it to our development team on your behalf. While we can't guarantee implementation of every suggestion, customer feedback is an important part of how we continue to improve our platform, so we appreciate your input.