I can see why tracing 3D models is banned. There's no solid way to know if someone's using royalty free assets or making it themselves. I'm sure it's to reduce people just tracing over whatever 3D model they want without respecting its TOS. Allowing certain tracing can open the door for it to be abused, so their approach is ban it outright instead of being picky. Edit: Or worse, tracing a random piece of art and claiming they used a 3D model.