So the adult argument has fell through. Now it's about being a customer.

Service providers can fire customers. If you pay for membership of a club but never attend functions, the club will end your membership.

If the service aims to provide a community or is invested in the success of its customers then it should fire customers who are not invested in their own success.

Not to mention how many students are not customers. They are sponsored by parents and scholarships and heavily subsidized. All these stakeholders have a vested interest in academic success. Tracking attendance is better for all of them.