>> banning future drones

It is about money. If they ban drones that are already inside the US, they risk lawsuits by drone owners/importers for expropriation of their property. Banning things that are not already inside the country is easier as nobody has an absolute right to import stuff.

It is akin to weapons bans. Banning future sales of machine guns is far far easier to implement than outlawing those already sitting in gun cabinets across the country. The former is free to implement, the later very expensive.