The Maginot Line
"... But is it legal? Microsoft evidently thinks that Google's Maginot Line can be circumvented..."
"Evidently thinks"? Not necessarily. It may be no more that a tactic to overtly illustrate the current anti-competitive restrictions being applied by Google.
Many might smirk with schadenfreude given Microsoft's history of similar market abuse but, as immoral as Microsoft were, the rest of us do not benefit from another company applying similar tactics/morality in the mobile OS world.