It could also be an exercise in open computing, hailing back to an era before legal attacks and ubiquitous surveillance.

Being released in 1976, the Z80 architecture is well known and unencumbered by patents. It's also simple with fixed instruction timing, meaning it can be well tested, leaving few places for a back door to hide. The original hardware probably predates any surveillance programs (edit: and the silicon is being publicly reverse engineered by enthusiasts). There's a satisfying feeling of control, when in charge of a computer that is simple enough to understand in its entirety.

A Z80 won't be the fastest computer, but it might be useful for some tasks. Updates to the Z80, starting with FPGA cores, will be faster than the original, and might form a basis for enthusiasts to develop further. Let's face it, ARM's roots are in Z80 era processors.