I understand it to work the other way.
I'm actually pretty sure that sniper turrets DO suffer the attack power penalty when under a friendly Force Field.
As far as I am aware, anything fired from under a friendly force field generator suffers the attack power penalty, including units that are force field immune.
The logic is that even though they are tagged force field immune, the force field still protects them from incoming fire, therefore they suffer the penalty for being under a force field.
Note that only force field generators have the attack power penalty for protected units, Force Field Bearers and Force Field Modules do not impose an attack power penalty on anything they protect.
Again, that's how I understand it to work, would need to actually test this.
edit: Just ran a quick test and yes, sniper turrets do suffer the 75% attack power reduction when underneath a friendly forcefield.
Which raises the question, is the engine damage of spider turrets affected by this attack power reduction? I don't have the knowledge to unlock them to test it in my game at the moment.