It all comes down to the basic physical limitations. Copying may be illegal, but there is no physical loss -- in fact the opposite, it is beneficial. We ideally want the economic system to evolve toward fully realising the resources available. That means copying is free, and everyone should benefit from it as much as they like. And production is scarce and has costs, hence commands a price in a market (and in a better form of market would then be used according to need).
Of course, we may or may not, in various ways, be able to reach that ideal. But a law -- like copyright -- that restricts us from using resources that are otherwise free is not justified in a fundamental way. It is there only pragmatically, to get the best out of practical limitations. It must not obstruct us from finding other ways.