Using BitTorrent is not illegal or unsafe. Bittorrent help user to transfer large files to thousands of user in a short period. Even Facebook and Twitter use BitTorrent to move files around.
But the majority of the people tend to use torrent for download copyright content available on public torrent site such as The Pirate Bay, KickassTorrents, etc.
Due to this, Torrenting activities are monitored in US, UK, Canada, Australia, New Zealand and in some other countries around the world to trace people who are downloading illegal content.