If I bought 2 turrets on the rear gunner section, I should be able to use them in the front as well. Why is SOE forcing players to double purchase just to use the same weapon on a different hardpoint a few feet away? Besides the obvious answer of a cash sink, I don't see a reason. Players are allowed to use universal NS weapons across all characters without need to repurchase per character...why does the Sunderer force this purchase mechanic?
Your complaint and your reasoning are two separate things. Being an SC-purchased NS weapon means that all Sunderers for all your characters on the your accounts may equip the weapon. That has nothing to do with the situations where two of the same weapon must be purchased for the same Vehicle. The Galaxy is another instance of this. It would make more sense to complain about Infantry being allowed to share applicable primary weapon pools and the sidearm pool.
Same reason you need to buy the same hornet missiles for each ESF, they don't care about you enough to change it.