I can't use Chevy parts in my Honda, but if Chevy started charging 1/10th of what Honda is charging for similar parts, Honda would need to lower prices if they wanted me to stick with them for my next car, or to use genuine Honda parts instead of third party parts to repair my current Honda. That's because Chevy's price would help recalibrate my expectation as to what kind of prices are reasonable, making Honda look overpriced.
OSes aren't as interchangeable as cars, but a Microsoft upgrade price significantly higher than Apple's upgrade price might convince more people to stick with what they have until it is time to buy a new PC and get Windows 8 bundled, rather than buying an upgrade, because Apple's prices lower the expectation of what an OS upgrade should cost.