It can be and it is, because past and current enforcement is inconsistent. The law is written without discriminating between .177 or 6mm, but the people enforcing the law does so through the lens of their own pre/misconceptions and circumstances.
In order for an airsoft gun to pass as a firearm in terms of fps and muzzle energy, it would have to discharge a 0.20g BB at 786fps. Remember, it might be written as 500 fps or 5.7J ME, but it's meant to be and enforced as fps and ME.
"The Bird of Hermes is My Name, Eating My Wings to Make Me Tame."