- Edited
Ok, so before we get started here, it's important to remember artificial gravity is common on these ships, especially since they are military vessels and it's better for the crew's bone density and muscular system to have the artificial gravity. Consequently, this means that the interior weapons have gravity exhibited on them, meaning that the motors that turn the weapons still have to account for gravity.
My second argument against a Beam Cannon turret is that the components used for it all have to be directly attached to the Centralised Beam Charger (The middle section of the beam cannon). The crew on the left and right sides of the beam cannon are actually controlling the left and right Beam Chargers. The components which connect all the Beam Chargers have the best efficiency at that size, where all of the beam chargers are directly next to each other. If you were to mount it all on a turret, the entire thing would have to swing around, not just the beam emitter. This would not agree with the first part of the lore, with the artificial gravity.
I do have a third, less public argument against the beam cannon which is more balance than lore. Let's suppose I did allow there to be a turreted beam cannon. This would mean that they can turn to aim at targets. However, as I noticed with the small Laser Beams, this also means you can focus fire the beams, which destroys parts extremely fast. Imagine if you could do that with the Beam Cannon. It's already got double the damage of the vanilla Ion Beam Emitter (which isn't a turreted weapon) and if it could focus fire, it would completely obliterate ships.
You may not take the time to read through all of this, but good for you if you do.