According to a statement released by Microsoft, the company supports cross-play between platforms and devices for partner video game companies who would like to enable the feature. This hints that it was Activision's decision to place the limitation on the Windows Store version, a claim that was confirmed by a source of Windows Central, but the reasoning behind such a move remains unknown.
Unfortunately, there is no workaround to make the Windows Store version support cross-platform play. For PC gamers looking forward to Call of Duty: Infinite Warfare, the only solution would be to stay away from purchasing the game on the Windows Store and instead buy it from Steam.
But it appears that cross-playing may be a set-back as players who purchased a PC copy of the Infinite Warfare game on the Windows Store on Windows 10 won't be able to enter multiplayer with players on other platforms. This means that the gamers who purchased Call of Duty: Infinite Warfare via the Windows store won't be able to battle online with Steam gamers. The system essentially lets players who bought an Xbox or Windows version of a game to get a free copy of the game to play on the other platform.
This piece of information is available on Activision's website, but there is no such mention on the Windows Store.