Looking around, I'm not sure that Steam is truly the only platform it's available on. Some stores claim to be selling Kalypso launcher keys, implying the game only utilizes the launcher and not Steam.
Topic on Talk:Tropico 5
I didn't add that originaly. I don't know exactly how the DRM works seeing as I received a copy directly on Steam. http://pcgamingwiki.com/w/index.php?title=Tropico_5&oldid=105740