Maybe to go one step deeper, the moral obligation is to speak honestly and fairly to the extent that one is able, and as their honest impression was positive, then the moral obligation to speak (honestly) positively follows from that.
Oh, of course - I don't mean that I believe Microsoft can do no-wrong: on the contrary, it (hyperbole warning) breaks my heart whenever I see Microsoft do something I feel is morally wrong or otherwise disappointing. Like seeing a loved-one go-off-the-rails, I suppose.
