This may seem counterintuitive. After all, ShareAlike is copyleft isn’t it? Well, no it isn’t.
Copyleft is, as its name implies, a reversal of copyright. It restores and protects the rights that copyright removes and makes alienable. The rights, not some rights.
ShareAlike takes a gift economy-style view of one of the effects of copyleft; the effect of quid pro quo. Copyleft has, as a side effect of its operation, the effect of requiring that people share their work in order to have access to other people’s work. They have to share, and everyone has to share and share alike. Hence the name of the ShareAlike module of CC’s licences.
But we can share and share our work alike without maintaining people’s rights. Linus’s argument that Tivo should be able to share their source without respecting people’s rights to actually be able to do anything with it illustrates this. For Linus, quid pro quo trumps people’s rights. Quite how he could have developed Linux if everyone else had respected his rights so little is a mystery for another time.
And we can use the legal mechanism that copyleft uses to protect people’s rights, reciprocal licencing, without using that mechanism to protect people’s rights. You could make a reciprocal EULA for a proprietary operating system that required you to charge anyone you give a copy to a hundred dollars and send ninety to the corporation that publishes that operating system. That would be reciprocal licensing, and you could share the software under the same terms your received it. So the corporation’s marketing department could easily argue that this is ShareAlike as well as reciprocal licensing. It is obvious that this is not copyleft. It has neither the intent nor the effects of copyleft. It would be very useful for marketing and political reasons for people who don’t actually want to respect the rights of others to be able to call this copyleft and gain kudos by association with actual copyleft, but very few people would be fooled.
All this said, ShareAlike has most of the practical effects of copyleft. So it is probably a reasonable substitute for copyleft at the moment. It is only when confusing copyleft with some of its effects leads people to ignore what copyleft actually does that it becomes a problem.
For example when people confuse reciprocal non-commerciality with copyleft.
NC-SA is not copyleft because the presence of NonCommercial restrictions breaks copyleft. It removes rights, and even in the terminlogy of ShareAlike it does not Share Alike with the originally licencing author. Arguing that NonCommercial ShareAlike restores some rights, that copyleft is just reciprocal licencing, or that you are still sharing the work does cannot make NC-SA seem any more like copyleft. NC removes enough of copyleft from SA to no longer qualify as copyleft.
Calling NC-SA copyleft is therefore a mistake. I am not saying that it is necessarily or always a malicious mistake. Far from it, it is a mistake that people seem to make from the best of intentions. But it is a mistake that people should avoid making and should explain when they come across others making it.