I wouldn't eat at the closed restaurant that took in lionfish from strangers. It probably is illegal for them to take food (for money or other consideration) from unknown sources, probably would be considered "commerce" rather than recreation, making you need a license, and from their point of view? Who are you? You might be trying to poison the other customers. Uh-uh.
Maybe in a more casual place, but in the US, if you want to give the fish away, you may have to grill them on the beach and hand them out yourself. Too many issues for a restaurant to risk their license.
I can understand wanting the spines to be cut off, but I wouldn't want to eat at a restaurant that let's someone else gut and filet their fish. Any decent seafood restaurant should know how to guy and filet a fish quickly and easily.