Microsoft believes (correctly) that most of its users don't understand much about computers. If you ask them "Which OS are you booting?" the _better_ answers will be something like "Microsoft" or "Office" and most people will just stare uncomprehending.
OEMs have consistently choked systems with shovelware because doing so is profitable and the margins in the PC world are beyond razor thin. So Microsoft's OEM license includes limits on shovelware, one of them ensured that when you buy a Windows PC, it boots Windows.
Because the alternative is it boots "AdBrowse - the great new way to earn valuable prizes while using the Internet" for which the OEM was paid $$$. And only a tiny fraction of users know to hold down Shift F8, pick "Windows" from the menu and boot the actual OS instead. Everybody else just complains that now "the Microsoft" makes them watch adverts, just as they queued up to complain that "the Facebook" had changed when they typed "facebook login" into a browser and got some blog page not facebook.com
More recently we've seen what happens when Microsoft's OEM negotiators let the OEMs bully them - that's what led to the Vista debacle. Every top-selling PC in Vista's opening season was unsuitable to run Vista, but OEMs didn't want these PCs gathering dust as "XP only" so they moaned until Microsoft authorised Vista badges of some kind for these machines.
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