system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: If torrents are not inherently illegal, why are ISPs allowed to throttle them?
gpt
ISPs are allowed to take measures to ensure the proper flow of traffic on their network, most ISPs don't specifically target bittorrent or even bother with deep packet inspection as the throughput of data makes that prohibitively expensive.  Rather, they target specific signs that are generally indications of torrents such as a large number of connections to are large number of different IPs or a large amount of overall data flowing to a specific IP and then lower the priority or cap the flow of data.

The claim is that this is done to protect other users on the network from suffering a loss of service due to a heavy user.  It may appear to target specifically bittorrent users but it can also affect large file transfers over FTP as well as some software update clients that use distributed updates.

So they claim they are protecting the function of the network for all users rather than targets a specific group of users.  

Generally most ISPs have a term in the terms of service that states that they may reduce or halt your service if you exceed a normal amount of usage, so they cannot be held accountable for not providing you with your full service.