Arguably, BitTorrent is mass distribution of, frequently, large numbers of files, by design. That being said, I think you see a lot more activity against the torrent tracker sites than individual seeders, and it's exceedingly unlikely someone who just leeches would see any sort of legal harassment.

The last part depends highly on your location. In Germany there are law firms specialized in watching popular torrents. They make millions from threatening users with legal proceedings and if you leech popular content you're more likely than not to get a letter from them.