At the end of the day all link building is unnatural and from Google's perspective is, to some degree or other link spam.
It can be seen from direct observation that article directories have nowhere near the power they used to have - however, when I started out we used article directories to drive traffic to our sites. After a while people got the idea they were good for cheap back links and then everything turned to spam.
So, here's a suggestion: think about site visitors, readers, human beings. Look at providing good quality, interesting and worthwhile content on sites that are visited by real humans. Look at getting traffic that is not dependent upon search engines. If you get real traffic from your syndicated content then search engines will come following because it is in their best interest to do so.
If you do something to get a back link then it is a spam link. The only question is how spammy?