I would not reject it, since it's still free software regardless of what the author called it. The author might call it open source because they reject the social, ethical and political underpinnings of the free software movement and instead think software should be free because of technical reasons.
Or they might call it open source because they don't know the difference: Some I know say open source but what they think it means is the same as free software. For those people, it helps to spend some time explaining the differences and then they usually start saying "free software." So there are two categories of people that say "open source." In fact, people may have many different motives for writing free software: http://www.gnu.org/philosophy/fs-motives.html but it's still free software regardless of their motives for writing it.
