In a few short hours, Call of Duty: Infinite Warfare and Call of Duty: Modern Warfare Remastered will unlock on Steam and Windows Store. If you're intending to get it for the multiplayer, though, then you need to choose your store carefully. According to Activision's support page, Steam users can only play with other Steam users, and Windows Store users can only play with other Windows Store users.
"We need to stick together! I wanted to pad my Gamerscore and you're the only one online!"
They do not elaborate on why this is the case. PC Gamer speculates that it could be an issue with Windows 7 versus Windows 10, but that makes basically no sense. The protocol between computers is just data, controlled by Activision, so the operating system that transfer it from network socket to game application is irrelevant.
I think I know what it is, though. According to the same support page, they note that a Microsoft Account is required to play online with Windows Store. I'm not sure if Activision voluntarily chose to use two different account systems, or if Microsoft pressured Activision to use Xbox accounts on Windows Store, but I'm guessing the incompatibility is due to Steamworks versus Xbox.
Again, I really don't know why Activision chose to, or was forced to, split their user base. We'll need to see if this becomes a trend going forward, though. If it is, I can see this hurting Microsoft more than Valve.