Copyleft, the copyright licencing reciprocity hack introduced by RMS as a core element of the GPL license, is built on top of copyright law. Providing source code of modifications of a GPL'd program is only an obligation when you distribute copies of the derived program. And only those people who received copies from you can demand access to the respective source code, under the terms of the license.
If we try to make a parallel to 3d printing, then we must ask ourselves wheter printing a physical object triggers copyright law or not. I'm unsure about that, Maybe copyright law is not as strong a machanism for implementing the copyleft concept in hardware.
Also, lots of people are using software/content licenses such as the GPL v2/v3/or later and CC By-SA for hardware designs and 2d/3d models for laser cutters and 3d printers.