That doesn't matter with a view on the game mechanic as such. You could argue, that in any game, scifi or not, an allied unit, especially one on guard, might warn an ally of an imminent attack.
You MIGHT solve the problem by allowing units to spend their action points in a different way; with ranged units that have 3 APs and 3 shots, spending only 2 on shots and one on going on guard would solve the problem anyway, but I think, this would only be more work AND dull things down.
At the heart of it, with the exception of some special actions (say, Defensive Strike), the philosophy is, that once you engage in any way you are vulnerable from behind, simply because your attention is on the engagement. You can debate about that, but it works, imo, in the sense that it gives outright attacking a downside. (I'm pretty curious on how Overwatch as a new tactical element will work out.)
Lastly, I also think that this helps balancing aoe-attacks a bit; ideally, you have to start with them, and once you have engaged they are difficult to use. This is, by the way, true for AoW3 as well (you probably just didn't notice it, but you can flank your own units as well, for example with a Draconian Flamer).
