During the initial days on Oshur, the Empire's leadership were a little too busy with expedition operations to bother with much in the way of warfare. With an uneasy, extremely temporary ceasefire in place, many of the troops deployed to the archipelago seized this rare opportunity for some R&R.

You certainly don't often see them without it, but yes, their armour packages are detachable. After all, modularity is just good engineering, and just because your skin's made out of semi-rigid ballistic nanoweave plating doesn't mean it ain't nice to air things out every once in a while.