Yes, the enemies are not protected and are under the force field, while your ships are under the force field and are protected but also doing less damage.
It's possible we need to introduce an "immune to friendly forcefield firepower dampening" type of immunity for the really short-ranged things like fighters. On the other hand, if you're on defense and you're building some turrets and what have you outside the forcefield, and if you see the bombers coming from a distance and they actually stop to shoot the forcefield, generally they are outside the forcefield and so are you. The more I think about this, the less I think this is an issue. YOUR ships are never firepower dampened by enemy forcefields (or vice-versa), so fighters on "offense" as techsy730 mentioned, are still just as powerful as before.
On the defensive side, there is NO reason to have piles of forcefields on your side of the wormhole on top of the wormhole. It literally accomplishes nothing except preventing AI ships from leaving your planet if that's what you want to do. But the consequence is that they're popping out inside your wormhole. Solution? Short range or other turrets that are good against bombers set outside the forcefields, slaughtering anything that comes in. And in what I've seen of them on the enemy side of force fields, the enemy almost always stops to shoot the forcefield well before getting under it (as I designed it to), making it so that your fighters can freely engage outside. The problem is when the AI ships start inside the forcefield to begin with, and then don't have a reason to leave. That's something that should be avoidable in most circumstances.