For an interesting read on the subject, I suggest the following:
http://www.dmlp.org/legal-guide/linking-...-materials
In particular, back when I wrote the forum piracy policy, I was being guided by Perfect 10 vs Google, 508 F.3d 1146 (2007), with my theory being that all an addon really is is python code pointing to and embedding a piece of content on the screen, not particularly different from any website embedding an image file. If the content were somehow retransmitted through Kodi servers, there would be problems (and there might debatably be problems if an addon provided a means of retaining a copy, as the Perfect 10 ruling doesn't cover that), but if all the addon does is display content from a server that has the copyright to display it, then Kodi is in the clear, from a legal perspective.
It's worth noting that the Perfect 10 holding does not differentiate between what is and is not easily accessible through regular HTML navigation of a server. It merely states that if someone is linking/embedding content from a server that legally has the right to distribute it, then such actions are permitted.
Meanwhile, the Grokster case makes it pretty clear that if you knowingly link to pirated material, you may be liable for "contributory copyright infringement." Since we all know that those piracy addons are piracy addons, Grokster is the legal reason we don't include them in our repo. It's also the reason why all those people online claiming those addons are "legal" are such hypocrites. They know that these addons are definitely not legal and if they host those addons, they are almost certainly liable for contributory copyright infringement, which is why they all tend to have their sites and places listed offshore and in various non-extradition islands.