Let's say I made a video. If I wanted to sell you a performance of that video and not your brother, and your brother manages to watch it anyway, fine, an offense has been committed.
But what if I sold the publishing rights to that video to a corporation who is bound by law to maximize revenue for my benefit? I've declared by releasing my video for public consumption that my interest is purely commercial.
That corporation does not generally have the legal ability to restrict who buys the rights to watch my video. Let's say the corporation decides to price viewings at $100 a pop. Your brother pirating my video and sharing it with his friends would arguably be doing more good for me than the alternative of fewer people watching it and me making a somewhat smaller amount of money.
Now the ethics are way more muddled. I'm not saying copyright is a bad thing or that infringement is a good thing. What I'm saying is that technological, cultural, and legal hurdles have made the content distribution market non-efficient and therefore non-optimal for content producers. It's virtually impossible for pricing mechanisms to maximize revenue, so there's always going to be situations where piracy makes sense because the content creators won't be able to make money on every viewing, but still derives a benefit from broader consumption anyway.