The BitTorrent protocol itself is a neutral, legal technology used for sharing large files. However, downloading or distributing copyrighted material without permission is illegal in most jurisdictions:
The rise of the "plug-in" and the illicit streaming device (ISD) has changed the game. For the average viewer, searching for a torrent, waiting for seeds, and managing a client like uTorrent is a technical hurdle too high. It is now far easier to use an app or an "unverified" streaming site to watch the show instantly.