Bittorrent normally hash checks all content against metadata in the torrent file so a simple MITM wouldn't be enough to inject malicious data unless the torrent metadata itself is being sent in the clear.
As an ISP if they detect you doing it, and they control DNS servers, maybe they mark your account for death, and they could like randomly hijack you going to google.com to some download for malware, and unsuspecting user clicks accept? Not sure, i'm curious how they pulled this off.