For the content of the Torrent, yes. Actually using a torrent client and downloading with it isn't illegal as it is a communications protocol. Quite a few companies use this for distributed filesharing across their wans.
The problem is, the average home user isn't going to be doing that with their normal data and will more than likely be using it for music, videos, etc. The assumption is made that these are illegal downloads and not your own or paid for media. Some ISP's are making it part of their terms and conditions and sending out letters based on traffic moving across certain ports on the assumption people were doing this, but it was nothing more. The letters were very cleverly worded. Some people along with this were having their contracts cancelled or bandwidth throttled due to high downloads and commonly used torrent ports open.
ISPs generally have no [legal] proof you are downloading copyrighted/protected media. It takes an awful lot of get a court order to monitor someone traffic directly as it covers quite a few separate laws. Sharing a few films every week isn't going to do it.
If you are going to use a torrent client, (for legal reasons obviously) use something like utorrent which can pick a random port for you or use one that is used by another protocol. This will prevent the port triggers at the ISPs. You can do a search and see which ISPs allege to monitor their ports.
Thanks Geoff