They are an extremely grey area in the law.
Unless you pay the tax stamp and the device is registered as a silencer, it would absolutely never be legal to use it as a silencer under any circumstances.
Owning the device and having an oil filter even in the proximity is dangerously close to being in "constructive possession" of an unregistered silencer. Having the device installed on your gun with an oil filter screwed on and having a hole in the end of the oil filter puts you absolutely positively in possession of an illegal silencer.
Frankly, I think the company makes them with the absolute intention of calling it *wink, wink* a "solvent trap" all the while knowing full well that most buyers will use it as an unregistered silencer. So long as no one actually says that's what they're doing *wink, wink*, they can claim ignorance.
It's silly. There's absolutely no reason to spend the money on an automotive oil filter for use as a "solvent trap", say nothing of the $75 device to attach the unnecessary "solvent trap" to your firearm.
I wouldn't consider owning one under any circumstances.