This aspect of the FSS turret is pretty much universally disliked, but here's a little experiment to show that it was a needed decision, that you have to be still (or moving very slow) for it to be usable.
First, find a system where you have some targets you need to zoom in on. Already-discovered systems should do nicely here, as you can check the system map to see the body configurations. (With a tinge of nostalgia.)
Once you've found some, target it and start flying away somewhere. Not directly at your targets, but do note their locations. Build up some good speed, so that if you zero your throttle, it'll still take some time for your ship to come to a (relative) halt.
When you've done this, zero your throttle, and quickly try to resolve your targets via the FSS. (Note that if you do this near a body that slows your deceleration down, you'll have more time.) Depending on your speed and distance, you can make it so you won't be able to: you should notice the blobs are moving, and they might even be moving faster than you can move your turret view.
So, there you go. If Frontier let us use the FSS whenever, you might be able to move in a way that you couldn't use your FSS. (I assume this would especially be true if you were accelerating.) So they decided to limit it instead. Perhaps it could be done via increasing the max turret speed, but seeing the quality of the implementation in general, they probably didn't have the time to fine-tune for this.
First, find a system where you have some targets you need to zoom in on. Already-discovered systems should do nicely here, as you can check the system map to see the body configurations. (With a tinge of nostalgia.)
Once you've found some, target it and start flying away somewhere. Not directly at your targets, but do note their locations. Build up some good speed, so that if you zero your throttle, it'll still take some time for your ship to come to a (relative) halt.
When you've done this, zero your throttle, and quickly try to resolve your targets via the FSS. (Note that if you do this near a body that slows your deceleration down, you'll have more time.) Depending on your speed and distance, you can make it so you won't be able to: you should notice the blobs are moving, and they might even be moving faster than you can move your turret view.
So, there you go. If Frontier let us use the FSS whenever, you might be able to move in a way that you couldn't use your FSS. (I assume this would especially be true if you were accelerating.) So they decided to limit it instead. Perhaps it could be done via increasing the max turret speed, but seeing the quality of the implementation in general, they probably didn't have the time to fine-tune for this.