Why should it be an either/or thing? Both are valid reasons for Google to prefer its employees use Docs. However the fact that you
can choose to use MS Office without any special permissions somewhat undermines your reasoning in point 1.
Google may be a rich company but it's also a very frugal company in many ways, particularly wrt technology (they pioneered the "huge amounts of redundant cheap hardware" approach to DC construction, for example). When Googlers were being coaxed into switching to Docs from MS Office, the financial benefits were front and centre to that pitch.