Actually this is how gyms work, they just dropped the pretenses.
thanks to Marginal Revolutions:
Peter Risager, a loyal MR reader, relays the following to me:
A Danish chain of gyms is now offering membership free of charge, with the only caveat that you have to show up, in order for the membership to be free. If you fail to show up once per week you will be billed the normal monthly membership fee for that month. This should solve the problem with incentives that gym-membership normally carries – there is suddenly a very large (membership is around 85$ per month) incentive to show up each week.
He offers also a link in Danish.