I just checked, the two most popular public torrent sites have COMODO CA default-on https.
These sites do want to maintain security, so their users can keep coming back rather than getting copyright strikes or worse fines and getting scared off. That doesn't stop them from putting viruses in the ad banners, but that is not getting their users arrested.
Torrent sites, especially the popular ones mentioned above, are almost entirely used for pirating. Bittorrent, on the other hand, is not exclusively used for piracy.
These sites do want to maintain security, so their users can keep coming back rather than getting copyright strikes or worse fines and getting scared off. That doesn't stop them from putting viruses in the ad banners, but that is not getting their users arrested.