So wouldn't these suppressors be prohibited in of themselves? (without even mounting)
One would think so. To be honest, I have no idea how they get into the country. I was shocked when I saw them being sold.
Maybe it is just ignorance on the part of the authorities or maybe the threading is not designed to be installed on a real gun thus they are not considered to be a real silencer. I don't know.
In my experience the airsoft cans would work quite well as a real silencer depending on what firearm they were mounted to. I'm not about to point that out to the government though. That is their problem not mine.