I don't think you should have to have a license to kill an invasive species, but it's a necessary evil. If they didn't require a license, every empty handed spearo would say to the warden, "Oh no, I'm just looking for LionFish, that's why there's no fish on the boat. Haven't seen any yet." I think FrankJig has the perfect idea! Kudos!
While duck hunting in NC, Game Wardens have told us in the past that if a Camera Man comes on a duck hunt or turkey hunt, he cannot touch or handle the guns, duck calls, or decoys unless he has a license and duck stamp. He can't even help hand you the decoys to setup. If we were to bring a dog handler to help retrieve birds, he also is required to have a license because his dog is considered part of the bird retrieval process. If you handled a speargun without a license, I'm sure it depends on which Fish and Game Officer you run into. Just gotta hope he's in a good mood.....and met his quota for the day.