There is no legal status to building a computer that is optimized to run an Apple OS. Build as many as you want.
The club is wielded by the licensing terms of the OS. example:
"Permitted License Uses and Restrictions.
A. This License allows you to install and use one copy of the Apple Software on a single Apple-labeled computer at a time." So unless the computer is Apple authorized (they do not actually build anything themselves) you will be running unlicensed software.
Apple decided some time ago that the money in their pocked was best extracted from your wallet by selling obscenely overpriced hardware and making the OS and updates a part of the deal. Microsoft has made a similar plan to stop charging for Windows OS going forward for many users. They plan to make money by selling you useless services, to which the OS will be your gateway, instead of overpriced hardware, which has not been selling all that well for them.