Windows is a software platform. If you want to write a Windows app, you must link to the proprietary operating system code and be subject to Microsoft's ToS and licensing policies. Microsoft reserves all right to decide the rules, and they simply chose to use the traditional operating system model; theoretically they could be more restrictive if their ToS covers it (if we disregard the potential consequences from MS having monopoly in PC OS market).
Mac OS X is a software platform. If you want to write a OS X app, you must link to the proprietary operating system code and be subject to Apple's ToS and licensing policies. Apple reserves all right to decide the rules, and they simply chose to use the traditional operating system model like Microsoft did. Just a small note that the situation here is a little bit different from iOS because you can write UNIX applications without directly linking to Apple proprietary code.
iOS is a software platform. If you want to write an iOS app, you must link to the proprietary operating system code and be subject to Apple's ToS and licensing policies. Apple reserves all right to decide the rules, and this time they made a rule that the apps must be installed through their App Store.
My point is that it is by choice, either voluntary or market pressure, that Windows and OS X allows installing softwares without restrictions in most cases (although this is not always true since there are certain softwares like drivers that Windows has restrictions on what can be installed). It is NOT an automatically granted right to write and distribute software on either Windows, OS X, iOS, or whatever platform you name it. It is mostly historical reasons that most PC operating systems have been that way.
The argument involving iTunes on Windows is ridiculous for that reason. It is Microsoft's choice not to put such restrictions by making corresponding rules. Hence, they have no basis on discriminatory treatment written anywhere in their ToS, license, et cetra. They could introduce some blanket rules like Apple did in their App store, but they wouldn't dare because that is very unlikely to give them any advantage in their current business model.
iOS has been using the tightly controlled model in the beginning. As long as the app runs on iOS, it must link to system libraries, so Apple can decide that it cannot be installed from a channel different from their App Store. Also, Apple has all rights to set the rules for App Store that 1. IAP for digital goods used on iOS must pay 30%, and 2. physical goods, tickets, etc. are exceptions. What matters is it is written for iOS and distributed via App Store. I'd say that as a whole counts as "offering a space and taking percent commission" in retail analogy, not just App Store itself. It is irrelevant whether Apple delivers IAP content or not. Heck, you aren't even forced to use one if you don't like it.
Web browser argument is a somewhat different case. It is not technologically feasible to enforce 30% rule while conforming to user privacy rules and offering web experience that users expect. Also, using web browser usually adds more inconvenience vs. having them done all in apps using Apple IAP; hence Apple need not care.
