Ask HN: How can torrents be legally used for distributing copyrighted content?

In most countries distributing third-party copyrighted content without appropriate rights is illegal. People downloading such illegal content using torrents are often accused of distributing copyrighted material because of how the technology works. While downloading such content is a crime in Japan(and possibly some other places), it is the uploading part which is a crime almost everywhere and which causes torrent users trouble.

When a party, for example Humblebundle, releases copyrighted content using torrents, the end user ends up doing two things.

1. Download content from peers and seeds(which may include official humblebundle peers and people like the end user who have already downloaded the content)

2. Upload content to other peers who do not have the complete content.

I have read humblebundle's Terms of Service and nowhere does it mention that the end user has redistribution rights for the content.

Staying with the example, if HB wants to help users legally torrent the content it can add a line in the terms saying the user can download and seed content via torrent. Perhaps such permission is implied anyways when HB is offering torrents on their website, but it certainly leaves a gray area which might be abused by copyright holders in other cases.

How can torrents be legally used for distributing copyrighted content without the end user ending up illegally distributing it to unknown third parties?