If the packaging for the opsys prominently states that it must be installed only on an Apple-branded computer, well, silly (lots of purchased copies are coexisting with other systems on non-Apple hardware) but legally enforceable.

Ludicrous. That's no more legally binding than the usage instructions on any other product label.

For example, DVDs with computer-accessible special features all say that a DVD-ROM is required, the special features won't work in a regular CD-ROM drive. What happens if you put one in a CD-ROM drive? It simply doesn't work. It certainly isn't illegal to do so; it just is pointless.

By your theory of product label instructions being legally binding, it would be illegal to put a DVD into a CD-ROM drive, not to mention illegal to use less than the recommended amount of detergent in your washer or dishwasher, illegal to not reapply sunblock every couple of hours, illegal to see if Quake 4 will run on a system that doesn't meet its minimum spec ...