Why should they? Then they'd be obligated to support all of these homegrown abominations that people come up with. Apple made the correct choice when not supporting the DIY crowd. Not that there's anything wrong with doing it, and heck I might even do it just so I can get my MBP back from my wife.
Did they? They certainly lost (didn't gain) the bulk of the PC market which you could argue is a good thing, but also relegates Macs to a much smaller (albeit more profitable per machine) niche market.
Apple's stranglehold on the hardware and software irks my hacker sensibilities, but it's the key to a much better user experience.
That said, I also don't see why they can't let the DIY crowd do it. Sell a version of OS X that is licensed to use on any hardware but not supported. I guess that would go against the perfect Apple experience, though.
Another poster says the structure of the program prevented the cloners from providing a sufficiently varied product - that may be true too.
However, Apple failed to certify more powerful clones (i.e. clones that were better/faster/cheaper than their big iron) on a reasonable timeframe. Cloners also contended that Apple was opening up the boxes to steal their trade secrets.
And the cheaper models -- that were often equal or superior to Apple's offerings, were a no-brainer for Apple's customers.
The long and the short of it is: Apple makes something like 30% profit off direct retail of their hardware system. Even on a mini that's pretty good. Compare that to $115 for the sale of OS X, and you realize they couldn't afford to keep the cloners around.
As I remember it, the opposite happened. Power Computing skimmed the cream off the Mac market because they had faster Macs than Apple.
Cloners also contended that Apple was opening up the boxes to steal their trade secrets.
That's an odd complaint considering that Apple gave them the OS, the ROM, and the chipsets to begin with.
UMAX complained because their C500 model later served to be the exact hardware loadout of the Performa 6400 Tower. Apple let them do "research" on what chipsets could do what, and then took the result for use in their own hardware designs.