Update: Valve has now taken it down.
This is sometimes the perils of open source, as there's a lot of people out their looking to make a quick-buck and they don't care who they burn in the process. To be clear, the version of 0 A.D. that has released on Steam (with it missing the second dot in the name), is not actually from the people who make the game — even though Wildfire Games are listed as the developer on Steam it's not them. Confusing right? Valve allowed it and approved it, so it does make me curious what legal checks are even done for this to happen.
I was notified of this on Twitter, with the official 0 A.D. account tweeting:
Another tweet sent today by the 0 A.D. team:
IMPORTANT: The person who released the game on steam today is charging 8€ for it. Please do not buy it. The game is free and always will be. It might contain viruses and other malware.
I imagine it doesn't have anything nefarious inside it, otherwise you really would have to wonder what Valve are doing…
Looking on the official game forum there's a topic asking about it, the developers seems to be at a loss as to what is happening.