Why put the spool on the drone? I'd think you'd put it at the base station, and maybe have a way to cut the fiber on the drone so you can pull all the fiber back to the base station. Ah, but maybe that makes it easy to find the base station, but unless the drone can be autonomous after some point and take the fiber with it far from the base station, I'm not sure the base station's location can really be protected.
EDIT: Ah, the real reason the spool has to be on the drone must be that the drone can't pull the fiber through snags, and if the spool is on the drone then it doesn't have to.
The drone moves, the base station doesn't. The spool goes on the thing that moves, so it just has to unspool to move further. If the spool was on the fixed position, the drone would have to drag thousands of meters of fibre behind it, needing more powerful motors and creating a risk of snagging/snapping.
Spool on the Drone means a a bulk of gradually decreasing weight, depositing the wire as needed. Spool on the station means the Drone has to pull the entire ever increasing length of the wire, which can get stuck or even ripped by the trees or other obstacles
I’m guessing the strain on the spooled end is less and you want the end most likely to detach close to the ground where you might be able to reattach it, vs in the drone where it would fall to the ground and be unfixible.
The whole concept is unfixable. Once the fibre comes out, it's not going to go back in. It's that deadly combination of fragile and cheap. Just unpack a new drone and off you go. Don't spend a week (and 10 casualties going into the grey zone to collect it) winding it back up only to find it's broken in the middle.
Cables drones would (and likely do) make better relays than as attack drones. A relay drone flies out to the horizon and relays the LoS signals to an attack drone flying over the horizon from the perspective of the base station.
The relay then gets to spend far more of its power budget on the relay-drone segment and has a very low power relay-base segment. Relay drones also perform surveillance while they're on station. Better bandwidth to the base station from a "wired" connection means a higher fidelity feed or potentially additional cameras to cover a wider area.
The consequence, problem is ending up with long strands of basically invisible fibers everywhere. But that is a problem to solve later. After the defeat of Russia.
EDIT: Ah, the real reason the spool has to be on the drone must be that the drone can't pull the fiber through snags, and if the spool is on the drone then it doesn't have to.