Well, that's a good point. I'm not really sure, they might decide to come out of the force field and attack the sniper turret. Or they might just send stuff from neighboring systems or via reinforcements to take them out. In some cases, they might just wind up getting whittled down over a very long period of time if there's nothing to defend.
That's part of the thing with emergent AI, it's hard to always know what it will do. If it turns out that I need a new behaviorlet in there to accommodate this new strategic option, I'll add it, but usually it's pretty good at figuring out stuff like this on its own. We'll have to see how that plays in practice over a number of games to really be sure.