That's a tough one. As you probably know Google changes the rules constantly. It seems to me that whether or not reviews show up has mostly to do with whether or not Google has crawled the page where the review is and updated it's index to include the info. It usually takes a few weeks from when the review is posted.
As far as ability to choose, sure they could, but they don't. Google like to make algorithmic changes rather than individual moderation, which does make sense given the volume of data they are working with.