bansama: That will be down to Valve's no cross-platform stance on F2P games. Apparently Akaneiro was coded to also allow cross-platform play (across Steam, their Web-client, etc.) but Valve nixed that. So if you want it on Steam, you now have to buy it on Steam.
To be honest though, it's an exceptionally bland game. Not really worth buying on Steam at all.
How weird that they would do that to Akaniero because Airmech has crossplatform play between its Steam, Chrome Store, and Desura versions. Not only that but you can redeem codes for promotional items directly from the developer's website and they will appear in your game across all versions. Purchasing in-game currency should work just the same but I haven't tried it yet.
Another game that supports cross-platform play is Spiral Knights. Unfortunately, unlike Airmech, once you link your account to Steam it doesn't let you use the in-game store in the browser version of the game so you can't avoid using the Steam wallet.