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.