The difference between mobile and PC is that if hardware maker has their own closed system, what they have is basically an appliance. PC's are generic item build from standards and Microsoft has produced an open operating system with large markets for software running on their OS. They can't just close it on a whim and abuse their monopoly power position [1].
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[1.] Findings of Fact in U.S. v. Microsoft Corporation, Civil Action No. 98-1232 (TPJ) and State of New York, ex re. Attorney General Eliot Spitzer et al., v. Microsoft Corporation, Civil Action No. 98-1233 (TPJ), November 5, 1999.
"Monopoly power" is not the same as monopoly. One must always point this out when saying it or discussion is derailed.