> intellectual property

There is no monolithic "intellectual property", it's an catch-all, umbrella term that refers to a large body of independent laws on copyright, patent, trademark, and trade secrets, which are historically established under different contexts. A work can be uncopyrigtable, but still restricted from reproduction due to a patent or trade secret status. It's why the term "intellectual property" should be avoided.

> Why couldn't the actual design of a PCB be copyrighted, for instance? I mean, one could argue that there's as much intellectual property being created when you route a PCB than when you write a book.

The bottom line is, just because something takes intellectual efforts to create doesn't imply it must have copyright status.

Copyright is an artificial monopoly constructed by the governments. Usually, the scope of copyright laws is limited to the expression of creative and artistic works, and furthermore, not the ideas or systems themselves, but merely their expressions. Many types of "functional" works are explicitly not covered under the copyright law. Also, it's also important to remember that the ultimate purpose of copyright law is to promote public interest. Thus it's entirely reasonable that the government deliberately decides not to put something under the copyright laws if the public interest takes priority. It's equally possible that the government suddenly wants to put something under the copyright laws because it considers the public interest is better served by granting copyright restrictions to the authors.

A controversial example is the French copyright law. The lawmakers in France apparently decided that the public interest is better served by putting the nightview physical appearance of the Eiffel Tower under the copyright law. In France, if you take a photo of Eiffel Tower at night and post it to the Internet, it's possible that your photo infringes the copyright of the nightview design of the Eiffel Tower. [0]

A better example under the U.S. copyright laws is rasterized fonts in bitmaps and pixmaps, they are not copyrightable, because the fonts are "functional" - it's more of an apparatus/machine for printing texts than a work of art. In the past 200 years, the copyright was never updated to include fonts, likely due to the concern that the lawmakers consider that the rights to the public to use the printing press outweighs the interest of the font authors. However, a digital font, a.k.a the computer program that generates vectorized fonts, is copyrightable as a computer program, but not the pixels it generates. [1]

Another example: a few weeks ago there was a news story about how a volunteer recreated and 3D-printed a ventilator valve and got threatened by the vendor. But if the case is ever brought to a court of law (it's not), the defendant's lawyers may argue that a single physical valve mainly serves as a functional part of the machinery, and by itself, it does not meet the threshold of being a copyrightable work by itself. On the other hand, the original CAD drawing of such a valve is protected under copyright. However, it doesn't stop someone from legally reverse engineering the valve and independently reproducing its CAD drawing, even if the CAD drawing is identical to the original file, you can argue in court that an identical drawing is the only way to express the idea of the valve. Unless the valve has a patent. Unlike copyright, a patent restrict the ideas themselves, not only its expression.

And finally we come back to electronics design. Most aspects of hardware design are not copyrightable, due to the same reason that the layout of integrated circuit wasn't really copyrightable until the 1980s. Back the 1970s, it was possible to produce a chip with the same layout from your competitor. The reason is similar to the font program vs font output: Although the pattern of the layout itself is a piece of artwork thus protected, however, the photomask it produced was generally not copyrightable because it's only an industrial apparatus for producing the chip. Thus, although the artwork layout in itself cannot be copies and redistributed without authorization, but it was legal to produce an identical mask for the chip, thus, it was legal to produce a chip with the same layout straights from your competitor. The only way to prevent others from making an identical chip was patenting the design processes or technology in the chip, however, many chips only use general-purpose technology and process, so it cannot be patented. The lawmakers decided that the U.S. public interest will be better served by allowing the masks to be copyrightable, so the copyright law was modified. [3]

As we see, although many aspects of hardware design can be copyrighted, like a CAD drawing or its physical appearance (as industrial design), however, a lot of other aspects of hardware design was, and still is uncopyrightable due to the same line of reasoning. Ultimately, stopping one from making a functionally identical machinery isn't inside the scope of copyright laws, but patent laws.

Another conclusion is that, while it's good to explicitly release a hardware design under a FOSS license, a FOSS license applied to hardware design is pretty powerless than when it's applied to software. Especially, copylefting hardware itself (You replicated our machines, release the source too) is somewhat impossible.

[0] https://en.wikipedia.org/wiki/Eiffel_Tower#Illumination_copy...

[1] https://en.wikipedia.org/wiki/Eltra_Corp._v._Ringer

[2] https://en.wikipedia.org/wiki/Integrated_circuit_layout_desi...