I definitely believe this is part of the business strategy for OS vendors. Microsoft actually successfully did this with new versions of Windows too, up until Vista (their incentives were less direct since they didn't produce the hardware directly, but they still got a royalty for every new PC purchased with a Windows license, which was all of them). Every new version of Windows would require a comparatively beefy machine to successfully run. Vista wasn't all that different in its requirements differential, but the market had started demanding devices that traded down computing power for more convenient form factor (the first generation of netbooks were big when Vista hit the streets) and expecting these devices to run Vista just as fast as they had run XP. Microsoft's failure to anticipate this shift in consumer behavior is mostly responsible for Vista's bad reputation.

Apple can continue to behave this way specifically because only Apple makes hardware that runs Apple software. If they opened up to third parties and someone created a computationally weaker but aesthetically preferable device that ran their software they'd be in trouble.