I read the thread you referred to in this one and I would offer the following comments on the matter.

#1 - Gnutella is a open network. Therefore, anyone can publish anything. This is obviously a good thing and a bad thing. i.e. Good = My company is a perfect example of this. Educational and Training videos / Applications created by LeeWare Development of course.

People who publish Open Source Software i.e. Linux/FreeBSD etc.
People who distribute movies/music/txt/videos from the public archive. etc. All non-infringing content.

Then you have, porn companies that advertise on p2p networks. fair use of the technology. (regardless of opinion)

Next, is child pornography - these people are predators and opportunist. --I include in this pranksters (immature kids).

This groups activities are illegal.

and Finally, software/movie/music pirates. People who largely deal in the distribution of infringing content i.e. stuff that they did not obtain any type of legal authority to distribute. Pranksters are also a part of this group.

This groups activities are also illegal.

The content published by these last two groups happens to be the most popular and sought after content on the network.

#2 Why do they do it?

Well, the first group that deals in child pornography and general pornography. Is made up of people looking for a way to anonymously trade illegal images and videos. Some times they rename the files to cloak their activies. However, the people belonging to that group know the true contents of the files.

Movie Pirates take ripped versions of porn movies and publish them on the network. (Copyright-infringement). The porn industry in response to this attempts to curtail the activity by flooding the network with video-clips that advertise the websites for such material.

The pranksters name the porn movies so that they look like infringing-content. Latest movie releases and unsuspecting users spend a lot of time download junk. The pranksters justify their actions by saying "they shouldn't have been trying to download x in the first place."

The irony is of course, people wouldn't come across such files if they weren't actively seeking them out.

#3 Why do people download this stuff?

Well, there are two reasons.

#1 They don't know any better.
#2 If they know better, they don't care.

There you have it. This is not a technology problem as much as it is a user population problem.

Hope this helps.

__________________
Lee Evans, President
LeeWare Development
http://www.leeware.com