Copyright law acknowledges that the marginal cost is near zero. That's why copyright holders get a temporary monopoly. The goal is to incentivize the creation of these goods so that everyone can (later) get them at near-zero cost. Copyright law does not enshrine the idea that "you should always pay for anything that is beneficial." The idea is that as a society we collectively want to always pay for something that is a net positive.

There are many ways this system is currently broken (e.g. copyright lasting 70 years after creators death in US), but the fundamental concept of incentivizing creation of works that ultimately provide public benefit is sound.