I've never worked at Microsoft, but have worked at other large software companies. This is 100% a design decision. Engineers like buttons, shortcuts, neat tricks, test tools, secret options, lots of menus, etc.
I also think this is a fairly sensible design decision.
I personally would expect dragging a file to the dock would put the file on the dock, rather than trying to open the file in whatever app you dragged it onto.
Initial designs, sure. When it comes to making trade-offs in features and functionality vs ship dates then that falls into the realm of a Product Manager.
