Actually all suppressors for airguns are illegal, unless you have the tax stamp etc. The reason mfg's get away with it is the ATF doesn't cover airguns so they are not concerned about them (not my job), but they are still technically illegal. Even if a suppressor has to be cut from whatever it's attached to doesn't make it legal, but many airgun suppressors simply un-thread and could easily be used on a firearm. All but one of my many airgun suppressors thread on, and I could put them on my 22LR in about 10min with duct tape, and they would work. The problem, imo, is a million things could be used as a suppressor so they only bust people if they can show intent. Like if you made a suppressor that clearly has no other function. So the muffler and oil filter on my car are both potential suppressors, but I'm ok as long as I don't mount them on a firearm, or have a mount ready to go. For example a shirt is ok until you wrap it around the muzzle. A solvent trap is one that has the use of being a solvent trap, but the ATF isn't buying it so they're illegal period. Even possessing the solvent trap adapter that connects a gun to a car oil filter is illegal, even tho it is not a suppressor w/o the filter. The gun is legal, the filter is legal, but the adapter serves no other function and the ATF is not stupid. So when people push the limits like they did with solvent traps, the ATF responds. I actually saw a full blown firearm suppressor on the Walmart website for $20. It had the right threads, it was an alum body with a hole on the other end and accepted a paper filter. Even w/o the filter it's totally a suppressor, but there it was on the website. I'd imagine they were either ignorant to what it is, or the "filter" is delivered by an ATF agent that asks if you ordered it, and will you accept it.
The Walmart link is broken but google Wix or Napa fuel filter suppressor. Ebay no doubt has them.
Here's that adapter I mentioned, which IS on the walmart site:
https://walmart.com/ip/148312318
Making a suppressor for an airgun no doubt puts you in a much grayer area, so I'd rekon that if you do it better not fit easily onto a firearm, which is kinda hard to do since they're so similar, and why the oem suppressors from my airguns will fit firearms.
Also note that it only has to reduce the report by 1dB, one time, to quality as a suppressor. So yes a lame plastic suppressor counts. I'd imagine fake suppressors count too b/c I'd imagine the tube length could drop it 1dB. Having a longer barrel that reduces >1dB is a-ok, just as selecting quieter ammo is, or a gas actuated gun where that gas usage drops >1dB. I'd imagine controlling all that stuff would be too complicated so they don't bother, but probably don't care either. The spirit of the law is to prevent bag guys from quieting their shootings so it seems they're just concerned with the obvious suppressors doing the obvious. Imo...
This link may help some people who think airgun suppressors are exempt for whatever reason:
http://tinyurl.com/zbtllh2
Many airguns don't really need a suppressor, but I want them on everything and imo a pcp is pointless w/o one. I went a step further with my Marauder and added a suppressor to the oem suppressor to pretty much make it a silencer b/c it literally is silent now. No way in hell I'd take that add-on off my property, but in my yard I feel safe enough to use it.