First of all, I would like you to read my comment to the other person who replied to me, because I did not state that anyone in this situation was actively guilty, only that the argument that people were not doing anything illegal solely because they did not actively do the act is overly simplistic (and in fact generally incorrect with regards to piracy, as my example from the other post shows, even if it is specifically correct for this example, although that would require some support). I will also ask you to please pay attention to who wrote the arguments you are responding to, as all language will be more easily understood if you know how to better decode the other person's statements.
However, you have now drawn a parallel to a different group of people (people who make clients, as opposed to people who build search engines), and I think the results there are more easy to see. For context, I run Cydia, the alternative to the App Store for jailbroken iPhones. We specialize (and in fact only really care about) software that is not an "App" (and thereby could not ever be submitted for sale in the App Store); however, as Apple rejects many apps, we tend to often get people coming to us with their submissions that they feel Apple should not have rejected from their store.
Many of these rejected products are torrent clients, and when they come to Cydia, we now blanket reject them as well. The reason is that the people who write these clients really are attempting to promote piracy in a way that the gun shop owner is not attempting to promote murder: the majority of their users are using them to pirate, they know that this is the case and are profiting directly off of the result, and prominently feature piracy in their marketing by showing screenshots of people pirating along with "see how easy it is?".
This latter point is very important: the firearm shop in your example was probably started by someone who either likes explosives, likes the history of war, likes hunting, or feels some need to protect himself and his community from the government or thieves or any other random threat; the person who owns the shop probably is not himself out killing people, he probably did not make the shop to make it easier for himself to get guns, and he probably did not build a business with the idea "most of the people I sell these guns to are murderers".
The result is that you don't see firearm shops that have giant signs on the front of their store with a picture of themselves killing someone as marketing for how simple and effective their guns are. When someone comes into their shop asking how to more effectively kill someone with their gun, they do not happily and cheerfully help them with their support issue. They also don't run community events (to draw a parallel to web forums) for people to mingle and discuss how much fun they have killing people.
However, this kind of thing is what happens when people start selling a torrent client: these two situations are quite obviously not the same situation, and yet I keep seeing this firearm example come up in defense of selling torrent clients as "if I can sell firearms that someone might use to murder someone, why can't I sell torrent clients that someone might use to commit copyright infringement". I seriously dare people to temporarily ignore the moral issues (as they are irrelevant for this specific discussion) and assume that murder and copyright infringement are equivalent crimes (alternatively, replace murder with something like "stealing candy from babies" or some other humorous and petty physical crime), and then think about how the mindsets and activities of these two actors can be discerned.
Regardless, the result is that you simply can't be successful selling a torrent client unless you pander to the pirates in your advertising and support because the pirates are nearly all of your user community; thereby, I found that without constant vigilance every torrent client I accepted ended up going down this path where I'd find the vendor's official Twitter account promoting piracy, the description on the products would keep ending up with more and more demonstrations of piracy, and I'd find out from users that they were specifically helping with piracy-related use cases via e-mail. Now, whether the law makes these people particularly "guilty" is another question: as someone whom these laws have occasionally threatened to smack down (and who has had friends actually get smacked), the position of Cydia is to simply not risk the result and reject the app (especially as you can always just host it yourself or with another party, and you can take payments for it yourself: it is a truly open ecosystem).
(Now, of course, I make this argument, and thanks to "howcan" and people like him, will have to add a giant hedge: nothing in this argument is trying to draw a parallel between the moral ramifications of taking a human life and copying a song; to the extent that you would like to have that argument you should not only have it somewhere else, but you should not have it with me, as the implication of taking that stance is that I am attempting to put myself in the "maybe I'm a murderer" category, which is both insulting and silly given what I do: it is the kind of argument that causes people who, in entirely well-meaning capacity, bring up an analogy from World War II, and then derail into "did you just call me a Nazi?!", when in fact nothing of the sort was implied by the relevant parts of the metaphor.)