Apple's Operating Systems could very well be considered a Monopoly.
The way this can be done is Apple is deliberately chaining it to hardware that Only they manufacture and market for their Operating systems. They are refusing to allow it to be run on any other platform.
Where as if you look at other operating systems, you do not see this happening so much.
MS Windows, Linux, and a host of other operating systems are designed to run on multiple platforms.
Granted, very few of them run on multiple platforms natively, but most of them come in more than one flavour so that they can run on what ever platform they are needed on.
So in a world where Monopolies are supposed to be illegal, how does Apple get away this or does the legal system think it's Ok because of all the computing platforms and Operating systems which are already present in the world?
Yet Apple allows Windows and MS Office to run on their systems on top of OS-X?
Go Figure!
