That is not the premise, that is the matter in question.
If it is simply assumed as the answer, nothing has been said.
As to the idea of harm here, it is not in the actions or materials themselves. It depends on assuming the particular law is the baseline, and then saying without that the person would be worse-off. That is effectively saying: people can make a living in a particular way because of the rule, and we have the rule because people can make a living in a particular way from it. This is a circular fallacy. It cannot be justified by itself.
Why is IP justified? -- as a rational argument, in terms more basic than the question itself . . . (having looked around a bit, there seems to be only one half-decent/plausible one: the standard economic one -- which is not really an ethical argument (and conspicuously lacks actual evidence anyway))