No chip manufacturing process has anywhere near 100% yields for CPUs, so this is just good business. Also, both Intel and AMD have released CPU packages with disabled features. It's simply a means of using a single production line to create chips to cover a wide price range.
Historically, slow versions of chips have often come from the same production run as fast versions, with testing used to sort the best from the worst. Incidentally, golf-ball manufacturers have been known to do this too.