Think of the 500 (or any index strategy) as being like a story and the methodology is like the book that tells the story. You can't rip off our story and write your own book that essentially plagiarizes the 500 and claim it as your own. If you were to, for example, write a story about a rich kid who's parents were murdered and when he grew up he became a vigilante who wore a disguise to conceal his identity and worked with the chief of police to fight corruption in his city, I have a feeling you might get sued. This happens in the movie industry from time to time. That's why script readers are exceedingly careful on what they read because there have been a number of cases where someone submits a story which then gets passed along but eventually passed-on and then a while later the studio ends up making a movie written by someone else who submitted a scrip with similar story elements and the studio gets sued by the first writer claiming they ripped them off and just paid someone else to rewrite their story. Sometimes it's true, sometimes it's not.
The reason this same legal principal applies to index products is because it's surprisingly difficult to even match an index's composition and weighting even with the methodology document in hand. So the odds of you creating your own strategy and that just so happens to be damn near identical to another index is essentially impossible. So just like the entertainment businesses, they look at it on a case by case basis and examine whether or not the "spirit" of the strategy has been violated or it's creative elements have been stolen. It's done case by case because it can get pretty nuanced and subjective just like music, books, and films.