30 Rock has a great scene making fun of this very scenario. It doesn't seem to be readily available online, although it's on the episode ¡Qué Sorpresa! for those interested. Then Forbes wrote an article explaining why this can never happen.
Edit: It is worth mentioning that Alexa being outside the TV does make a difference, as it's unlikely to have a directional microphone as would likely be installed in the TV.
http://www.forbes.com/sites/briancaulfield/2012/01/12/hed-wh...
Edit: It is worth mentioning that Alexa being outside the TV does make a difference, as it's unlikely to have a directional microphone as would likely be installed in the TV.