I thought it depended on the state in the US?
Here in Australia it is state dependent. I know it's legal in Victoria and Western Australia and illegal in New South Wales. Not sure about other states.
People get all uppity about spearfishing on scuba, but really, it has its merits. I mean if you're in a region with little coastal reef and sharp, deep drop offs, you may not have any other choice but to spearfish off scuba. I mean, if all the fish in your area are at 40m depth, good look freediving down to that depth, let alone staying long enough to hunt. I mean, you could berley up at a shallower depth to try and draw some fish up, but it's not always that easy.
Morally I don't see the outrage. Freedivers get all offended and say it's not fair. Realistically whether you're freediving or diving on scuba, either way you have a gun. I mean you're using technological advantages either way. Why is one fair and the other not? If you're hunting for food rather than sport, who cares about fair anyway?
I've done both and they both have their merits. freedive spearfishing is a lot more fun though and generally lets you cover more area.