People tune in to watch people sing, not voice political opinions. Well a bit here in there during the artist's backstories is understandable, the show is not an appropriate platform for anything beyond that. If singers or coaches want to express political views, they should do it on their own time.
Politics is a rather heavy and, especially recently, an unpleasant topic for a lot of people. The Voice is a show about singing and it is meant to be an entertaining show. I do not think people would be all too thrilled if they had to sit around and hear about politics.
The Voice audience might not want to hear politics while watching the Voice, but that does not mean that people should be prevented from expressing their political views. It is up to the producers whether they want to air statements that have a political message or a controversial message during the show.
People already hear about politics in nearly every other aspect of their lives these days. 'The Voice' is a singing competition, and people watch it for relaxation and entertainment. With politics present everywhere else, I think 'The Voice' should cool it on discussing politics and focus on the real reason people watch the show.