Here's a cut of what the developer said on the Steam forum:
It's on the way.
The devs are working on the Linux build and we hope to release it soon.
Please keep an eye out for updates!
I'm glad to see a Linux version wasn't dropped for this, since it was confirmed to be coming to Linux for a long time, including in their Greenlight campaign on Steam.
What I find interesting about this game in particular, is that it was created out of the experience the developer had during the NoTAV protests in Italy. Instead of focusing solely on one side of a conflict, the developer allows you to experience both sides. It includes a campaign and a versus mode to play against friends or the AI and they are also planning a level editor.
It sounds like it's a little on the rough side right now, considering it's in Early Access and apparently a little on the buggy side.
About the game
RIOT – Civil Unrest is the highly anticipated real-time riot simulator that places the player right in the heart of some of the world’s most fractious clashes. Campaigns include: Indignados (Spain), Arab Spring (Egypt), Keratea (Greece) and NoTAV (Italy). Seventeen single level scenarios include Rome, Oakland, Paris, London and Ukraine.
Thanks for the tip Michael!