If you need reasons as a LAN Admin to block torrents from your LAN, here's a few and ways to go about it (partially lifted from @Rory Alsop and @Polynomial 's answers here):
1. Exposing LAN to Malware/Exposing LAN in General:
This is an obvious one, and one that you're aware of. If you allow users to torrent files there is no way to ensure that they are using "legal" sources, thereby putting your LAN at risk. Also, if you can torrent, you've got some open ports ripe for poking. Blacklist all the "big" trackers and websites. Disallow the usage of BitTorrent clients. Lock down user access. Etc...
2. Bandwidth Concerns:
If Malware isn't enough to justify the restriction of BitTorrent on your LAN, then you could try from the angle of bandwidth concerns. P2P file sharing takes up a lot of network resources, thereby slowing down the network that is necessary for day-to-day operation (I'd assume). Limit users bandwidth if possible, or investigate those who are taking in a large amount of traffic.
3. This is YOUR Gorram LAN:
If the first two concerns aren't enough justification for everyone to stop torrenting, gently remind them that YOU are the Network Administrator, and your knowledge and expertise are the reasons you are in the position that you are in. Your knowledge and expertise has led you to the decision that BitTorrent compromises the network, regardless of their sentiments. Implement a policy that disallows BitTorrent usage, and enforce these policies with no leniency.
If this is a working environment then there shouldn't be any use for "shady" torrents. If there are files that need to be shared and your clients claim that BitTorrent is the only method, then put those LAN Admin skills to work and get some file shares going!
As far as reasons for not using torrents without a VPN... Well the VPN would only mask your IP address, and since your local governing body doesn't take action against illegal use of the BitTorrent network, then you have no concerns outside of the ones already listed.
I hope this addresses your question, please let me know if I'm on the mark or anywhere near it. Your question, as I understand it, may need some modification. Possibly a title like "Justification for disallowing torrents on a company LAN" or something of the sort. Just a suggestion :)