I hypothese there would be two reasons:
- because piracy enables more consumption of some media than the person otherwise would have done, it means they have less leisure time available to spend on other media which they would have spent money on. E.g. instead of buying a book and reading it, they would watch a low-value movie they pirated, affecting book sales but not movie sales
- because piracy decreases the perceived value of an hour of entertainment, the person would be less willing to spend money in other categories as well and would rather fall back to cheaper options
The result would be that the amount a person spends on _total_ entertainment drops, but the drop would be in other categories than the category that is piratable.