Actually a lot of popular app icons do have letters.
There are no soldiers or guns in WZI
The first and most basic resource in WZI is "armies" which is what's depicted in the logo. I'm guessing it was made that way to make it distinct from the WZ logo rather than just having the same logo with an "i" or "Idle" thrown somewhere on the logo to distinguish the two.
It's tough to make a good WZI logo that would accurately catch the essence of the game and not the theme. The theme is global domination of a given map through military might, but this doesn't accurately depict the essence of the game, especially one that simultaneously distinguishes WZI from WZC which has the same theme. The essence of WZI is managing resources and waiting for the right time to make a move. An hourglass and a Thinker statue is likely a better indicator of this than soldiers, but doesn't seem very exciting.