When you have marketed operating systems and software for over two decades in which clicking on an X at the top right corner always closes a window, program, application, or whatever else it appears in, with no further action taken by the process (without some sort of warning), and then, suddenly and uniquely, clicking on an X at the top right corner of a window causes the installation of a new operating system to begin (without any warning), this is deceptive marketing.
Microsoft's use of the X at top right corner of the Windows 10 installation dialog box (i.e., allowing the X to have the same effect as an OK button would) ... is not only deceptive, but also contrary to their own published documentation on guidelines for creating dialog boxes.
The guidelines can be seen on this page, and they state this ...
"The Close button on the title bar should have the same effect as the Cancel or Close button within the dialog box. Never give it the same effect as OK."
The same guidelines also state this ...
"Use Cancel or Close for negative commit buttons instead of specific responses to the main instruction."
Microsoft appears to be desperate about getting Windows 10 installed, and is willing to violate even their own guidelines ... this goes way beyond just plain deception.