I had the same thought, I even assumed there would be some international wartime law against it. But turns out it's apparently ok according to this study I found from US Naval College
> Conventional IHL (international humanitarian law), then, indicates that at least one form of disguising a military object as a civilian object—camouflage—is a permissible ruse of war, not a prohibited act of perfidy.
Unfortunately, my first though upon seeing military installations camouflaged as cabins, barns, homes - that gives an invading enemy a good excuse to target civilian structures...