You're falsely assuming bittorrent is only good for piracy. It's ideal for distributing larger - legitimate - files, such as games and game updates, which can easily be 50 - 100 GB. While those could be served by e.g. a cloud edge provider, the publisher would still have to pay per GB. Offloading that load to multiple host locations and clients will save a lot of money and network capacity. Imagine a game on launch day that has been bought a million times - not a stretch of the imagination. That's a hundred petabytes of data that has to be distributed across the world on launch day for a 100 GB game.