GPL is prohibited on WinPhones for other reason: license incompatibilities, which for me means that they were honest (as Apple weren't). I do know why MS wants to have an incompatible license with GPL as was threated by it. And I do think that is a bad move. I don't think anyway that this stops you to use Mono to create a GPL application, game or whatever.
If DirectX is proprietary, does Java is not? Or Cocoa in iPhones? Or your proprietary NVidia or AMD driver? I do hope that once NVidia will show pity to opensource and AMD too, to not talk about VIA, Apple, Intel (that they don't opensource IntelCompiler, because they are also enemies of opensource). If your DX argument was about WinPhones, I think you used Android (that have Android toolkit, no Java one like Swing) or Cocoa from iPhone. I do think that Nokia was the exception on this respect having all stack opensource. Will you buy fast Symbian Nokia phone for it? Just to see that it makes a lot of sense this opensource? Or you just wish that all to bow to your kind wish to have everything opensource and to open all sourcecode of their code. You may ask any corporation to give to you this openess and you will see that some opensource initiatives died (like Sun's Looking Glass project) just because people don't contribute back. I do work in an (hopefully free) opensource project and I do know its challenges. Is not that I don't like it, I just think that people confond always opensource with free and rarely (if ever) they contribute back. One guy here likes GNote and hates Tomboy. Why doesn't he contribute to GNote? Or better to make a much better Qt alternative to it (or contribute to a similar GNote implementation)?
I wish that opensource to succeed, and Mono at its core it is opensource (look that to benchmark a newer Mono, someone on this topic recompiled Mono to latest version to see if are performance gains or not).
Am I zombified? Don't you create a straw man?
Should I make the same argument on Ubuntu development? That they rarely contribute back? Or about Oracle? That they did not opensource their OpenOffice extensions? Or that JavaFX was not supported on Linux in 1.0 release and a part of development was closed? Or that Google have an "embrace, extend, extinguish" Linux kernel that its customizations were removed out of tree because they did their proprietary extensions so phone drivers can make easier their phone drivers but no opensource GPL way?
Will you try to push people to not buy IPhones, Android Phones, WP7 phone but just dumb phones, because there is no way to push those companies to opensource their platforms?