Apples system of doing things works for it and works against it. Its like the stock market, its risky.. if doesnt pay of, it could be terrible, if it does pay off, its going to make much more money than anyone could have thought...
Think about it, not only do they sell their OS, but they sell their hardware. Each item is marked up by a certain percentage, so they're making profits on the hardware and the OS. If they only sold their OS and realied on others to sell the hardware (of which they would see no returns[like microsoft]), with their current market share, they'd be finding it very hard to survive! Although, given, if they relied on others to make their hardware platform, their may be more choice, lower prices and many people may buy their software.
But if Apple makes the best computer in the world, and people continue to buy it, the prices go down more people buy it and may one day Apple wil lhave 50% of the market. If they where ONLY selling their OS, they'd be making much less profit!
Theres advantages and disadvantages. Apple are too far ahead to change platforms, and their platform is working for them atm. Small market shar, but their still profitable!