Copyright law is based on a great deal of legal mumbo jumbo. Patent and trademark law are similar.
They all have one very important feature in common. If the copyright/patent/trademark holder is aware of infringement and does not act, they can lose the ability to protect their assets later because they've now set legal precident against themselves.
This makes anyone with potentially valuable assets extremely protective. If you use a Metallica song for example, and Time Warner knows but doesn't come after you. And then they come after me for doing the same thing, I can argue to the judge that they didn't go after you. Now obviously it's not quite as simple as all of that, but you get the idea.