A merchant is entirely within their right to send you to collections and pursue you personally. There is no legal language in any merchant or processor agreement that prohibits it.
Gym memberships involve a contract and recurring payments, and the contract is usually not in your favor. A one time charge to an Uber driver is not similar.
You've agreed to a license agreement and terms of service when you use any app. You've read the agreement, right?
The lack of an agreement wouldn’t help you anyway. It would be theft of services, no different than taking a cab and running off after refusing to pay.