I can't agree with this. Affix does not imply it is permanent, you can affix any climbing aid, platform, camera, whatever, removed them 5 minutes later, you've still affixed them. I think this one is pretty black and white.
The wording varies state by state. Here is PA it boils down to not "damaging the tree". Vague, obviously even more so than the state above, but I think just about every warden is going to write you up if he witnesses you climbing a tree in gaffs. Or maybe, just maybe if he catches you with spurs and can link you to trees damaged by them. Both probably long shots. I think if a warden sees you with spurs at the truck, you may have to talk your way out of it or go to court, but that ticket definitely shouldn't hold up because how can he proved beyond a reasonable doubt you damaged a tree if he can't find the tree you damaged?
So there is the legality, and I think most cases on public land they are illegal. I have little doubt they are illegal here. Then there is the ethics. There may be some justifications there in relation to the actual damage and comparison to what other hunters do, but justification doesn't work in court. I'm not saying I am above the law but if the intent of the law is to protect the value of the trees, and I regularly see other hunters break the law, it becomes sort of like doing 5 or 10 mph over the speed limit. Pay the ticket if you get caught, and don't be an idiot and go spurring up trees in easily accessed places, or spurring trees multiple times. I can tell you after a month of backyard practice, it does do some pretty good damage. I think 3 or 4 times up a healthy tree and you're starting to put that tree in the danger zone.