An interesting legal issue here is that probably many 3-D designs are not copyrightable. Copying machine parts and similar functional designs is legal in the US, the UK, and most industrialized countries, except in the narrow cases covered by patents. Sculpture is copyrightable, but gears and bottlecaps are not. I'm not sure if there is explicit caselaw on this, but almost certainly parametric OpenSCAD designs are copyrightable, being textual works of creative expression in the same vein as Python programs or TrueType fonts; not sure about STEP/IGES parametric designs, although logically the same principles should apply.

So most Thingiverse designs that are copies of existing products are probably entirely legal, or legal except in a very few countries where they infringe Saudi laws about nudity depictions or patents or something, but by the same token, in many cases the licenses of the Thingiverse Things will not be usefully enforceable, because the STL (or perhaps a resampled version thereof) is statutorily not subject to copyright.