This perhaps illustrates the impossibility of legislating morality.
1. Yes the penalties for possession of prohibited gun accessories draws extremely stiff penalties in the US, and probably several other places.
2. In many places in the US it is legally required to have an exhaust spark suppressor on small gas engines used on lawn motors, string trimmers, chain saws, all terrain vehicles and anything else with a gasoline engine that is likely to be used in vegetated areas. Makes perfect sense as wild fires are a huge and growing problem.
3. When unmounted at least some types of the two devices, one legal and the other illegal, are indistinguishable. There is nothing other than custom that makes the mounting methods different.
So, in principal at least, it is impossible to tell whether a crime is intended or committed until the device is mounted somewhere. But I am sure the US law forbids the sale of such devices. I am sure this line of thinking is behind the ads you see even though these devices probably are terrible fuel filters. The vendor can get away with it, and and possibly the purchaser until the device is actually used.