As the transportation tickets are normally negotiated between the student body and the transportation provider it whats mandatory and whats not wholly depends on the negotiation skills and priorities of the student union body of the institution you're at.

With no opt outs the tickets simply get cheaper. My former institution allowed opt-out and before had complicated mixed system of buying a base tickets which allow travel on weekends only and buying monthly tickets for a little more than 30 EUR/month. After hanging a vote of all students this changed to a flat fee per semester.