To be in the swarm monitoring they must be downloading illegal material too
And you don't even need to grab the .torrent if you've got it's infohash. Then you can simply ask the tracker for some peer IPs... you can then contact the peers and do normal bittorrent protocol communication to see which blocks they have, use extensions to get even more peers, etc. But you don't need to ever get any actual file data.
Of course, if you want to verify
the a peer has the data it says it has, you'd need to retrieve blocks and check if their hashes match, which requires the hashblocks from the .torrent file.