How can one assume the viewer is only female?
It is because they provided a captial letter "A" and the they did not provide a capital letter "O". If you wrote "A espectador" that would be wrong because you are mixing the masculine noun with the female article.
But there's nothing wrong with 'o espectador', and yet that is marked as wrong. Another exercise to be reported...
I share the disagreement with Carosoza.. looks like a defect