Services like Shapeways do use SLS, though, and allow you to upload your own files for purchase. It wouldn't take more than ten minutes of effort to extract a Shadow Leech file from Stud.io or another program, scale it to the correct size, and upload it.
Counterpoint: because of their small size, Shadow Leeches would be one of the more cost-effective things for scammers to make fakes of. And I imagine Shadow Leech prototypes are less well-documented than the more sought-after collectibles like Kanohi and Kraata, making it easier to pass off a fake.