Far worse than this though, are the authors that end up pigeonholed because of assumptions regarding their potential audience.
Bernice L. McFadden, an author whose work and blog I quite enjoy, said a few words in this op-ed about the industry's marketing of African American literature, amongst many other salient points, that is quite worth a read.
A brief quote (emphasis mine):
This suggests that our literature is singular and anomalous, not universal. It is as if we American authors who happen to be of African descent are not a people but a genre much like mystery, romance or thriller.
That "not universal" bit is what really gets me going whenever I think on this problem. Again, why is it thought we can all relate to the assumed default whether or not we are part of that default but not to the many, many other people in our society?
A little tangential to this topic is this very interesting article by Matt Thorn regarding signifiers in anime. He reflects on "unmarked" and "marked" categories in different cultures.