And the answer is: if it isn't, neither is the (L)GPL. Why would MS not be able to place restrictions on distribution of their own copyrighted code?
Because those restrictions may not be legal. The (L)GPL does not actually place restrictions upon you, copyright law means that by default you are a lot more restricted over what you can do with any piece of code. Copyleft licenses grant you abilities above and beyond what you'd normally get. In contrast, EULAs attempt to place additional restrictions upon you, for which there are no legal precedents.
Besides, the idea that software is "licensed" is a rather legally dubious one anyway - for all intents and purposes it's a product, the idea of the license agreement as an after-the-fact contract has never (as far as I know) been fully tested in court.
thanks -mike