Again, I think this is where constructive intent might come into play. As it relates to SBRs, there's always been a ton of confusion and argument, but the generally agreed upon stance is that you don't have a problem unless you only own parts that could be assembled into an NFA configuration. For example, there's no problem with owning both an AR pistol and rifle...while you could theoretically assemble an unregistered SBR with the parts you own, you have the benefit of the doubt that you just own two separate, complete, and legal firearms.
Going to a store and walking out with a pistol upper and a rifle lower, while not technically illegal, might give the impression that you intend to mate the two once you get home, and you might draw some attention.
I've always assumed it was kind of a "tack-it-on" charge when they want to throw the book at you, but the solvent trap/fuel filter thing has always seemed pretty blatant. I'm sure someone somewhere out there has bought them and used them as nothing more than a solvent trap, but a solvent trap doesn't need baffles. I don't think anyone is buying them not knowing that you're a drilled hole away from something illegal. If there was a legitimate use case for them with no legal conundrum I'd think you'd see them on the market from all the major cleaning companies too rather than only being available from foreign sellers on questionable websites.
If someone were going to buy one and use it as a solvent trap as advertised, I'd think they would be wise to have one hell of a cleaning bench setup with a plethora of bore/barrel solvents and a safe full of spotless rifles to be ready to argue that they are a maintenance fanatic and their newly acquired solvent trap was just another piece of kit to aid in their OCD cleaning tendencies.