Starting a Question with "Why Not . . ." Inappropriate?

Microsoft Word has underlined a question I typed starting with "why not." Something along the lines of:

Why not walk to the hall every morning before the sun has risen and while the streets are empty?

According to Word, this is a non-standard question.

Is it correct in a semi-formal article that's intended for the Internet audience? I thought it was standard enough.

Solution 1:

It's a "non-standard question" (whatever they actually mean by that) in as much as it starts "Why not", which would make it into a rhetorical question — one which either doesn't actually require an answer or to which you will supply the answer.

Unfortunately, automated grammar checkers can only cope with a limited set of rules; and they flag items which might require your attention (like this, perhaps) in the same way as things which really do require changing.