They don't need to be exonerated. There's no reason to turn enthusiasm for open-source into a feeling of entitlement that all software must be that way. Some of the best software I use is closed-source. I have enough complaints about Firefox and Chrome that if a better closed-source browser comes along I'm going to use it.
I think they might, seeing the recent trends.
And forget about Linux - if extensions are going to get distributed through the Windows Store, as this article claims, you can say goodbye to OS X support as well.
Along those lines, you might expect them to make a Linux version of the Edge browser, except that it won't be the exact browser the majority of users will be running - I suspect they'd point you to the VM images they already have at modern.ie instead.
Is it that much of a stretch to say that Microsoft is obsoleting .NET? Given the moves in "modern Windows" and WinRT, with their focus on HTML5 and the revived interest in C++, plus killing Silverlight / WPF which was the hot new thing only 6 years ago, well, it doesn't look good.
Not only that, they just released a cross platform Code Editor which works on Windows, Linux and Mac, called 'Visual Studio Code'. That and more was announced in the Build Conference held recently.
[1] https://atom.io/
I'm tired of this trend. In my opinion saying you're using some piece of open-source library for XPath does not exonerate you from releasing a browser that isn't open-source in a market that has Firefox and Chromium. I'll bet they aren't going to release Edge for Linux either.
Microsoft has been copying Apple lately but Apple is the wrong model for Microsoft to copy.