The steam api is integrated into game. The api protects the game against distribution to other platforms and adds functionalities to it. I thinks it's also against the legal agreement to put the game with the api in this store. However, since you can claim a free steam key on this platform its not that unfair.
> The api protects the game against distribution to other platforms
Well, I bought the game on itch.io because it was DRM-free. It's a feature for me, I don't use DRMed stores at all.
> and adds functionalities to it.
What do you need it for, besides for multiplayer? It's a pity you need to rely on lock-in functionality which prevents you from releasing the game in DRM-free stores.