So, actors, musicians, and other entertainers don't like it when the Public tells them they should "stay out of politics"-- meaning that they shouldn't express political opinions. I mean, how dare they, really? You (the entertainer) have the same right as any other individual to say what you think-- just like any other person out there. Okay, I'll agree that no-one has the right to tell any individual that he/she shouldn't voice his/her political opinion. However, let's be honest about it-- Mr. Big-Name Actor expressing his ardent support for one side or the other of XYZ Controversial Issue is a little different from Mr. Small-Town-America doing the same.
However little it is usually deserved-- however little sense it makes-- Mr. Big-Name has more power-- a louder, farther-reaching voice-- than the average person. His opinions will reach a much larger number of people than Mr. Small-Town's ever will.
Many people lend too much credence to the words of the famous. Simply because Big-Name said it was so, the weak-minded will be swayed to support whatever he supports. It's another way of being cool like Mr. Big-Name-- like going out and buying a brand of jeans just because Big-Name was wearing a pair on the latest magazine cover... or wearing a cologne just because Big-Name's, er, name is on the bottle. It's unlikely that anyone will vote for Ms. Candi Date just because Mr. Small-Town writes about her on his blog, but if Big-Name tweets about her, it's a possibility.
Then there are a couple of other aspects of the issue...
Compared to the proportions in the country as a whole, it always feels like the left is (way) over-represented in the pool of entertainers-cum-political activists. If there were a more even distribution (across the political spectrum) of politically loud-mouthed actors, singers, etc., I think you'd see less complaining about entertainers with opinions. I'm not saying this means that left-leaning actors are obliged to keep quiet for this reason alone... Just that it makes a difference in people's level of tolerance for opinionated, liberal actors.
As an entertainer, your job is to entertain the Public. Ultimately, the Public writes your paycheck by buying your music, going to see your movies, and so on. The Public is your employer, and most employees find it beneficial to stay on the boss's good side (or if that's too unpalatable, look for a new job). So while you're perfectly well within your rights to spout controversial opinions, it may not always be in your best interest. Want to be successful in your career? Maybe you shouldn't actively alienate a large portion of your audience. Sure, it's your right to say what you think-- and to use your status as Mr. Big-Name Actor to make sure it's heard beyond your circle of friends-- but it's the Public's right to boycott your movies, albums, and TV programs if we don't like what we hear. A certain percentage may find it-- and by extension, you-- distasteful. They may not like to think that when they give you their hard-earned money, they're supporting your side of XYZ Controversial Issue though you.
Personally, when I'm thinking clearly, I try to avoid learning too much about entertainers I enjoy, because all too often, I don't like what I read. I don't want to know that they spend every spare moment supporting something I find abhorrent, because then my enjoyment of the music or TV program is tainted-- and sometimes, I'll even refuse to watch a movie simply because the actor's beliefs bother me that much. Maybe it's the same for a lot of people. When they say that actors shouldn't get involved in politics, what they're really doing is begging actors not to ruin this good thing they have going. ("Please? I really liked you in Edge-of-Your-Seat Action Flick III. Don't make me regret naming my pet hamster after you, okay?")
Entertainers (including some pro athletes) enjoy special status in our society. It comes with certain costs-- mostly in the lack of privacy-- but that's part of the deal. In exchange, they enjoy fame, fortune, and a multitude of perks. Also, for some reason, when the average person complains about how unfair it is for Mr. Professional or Ms. Big Business Owner or Mr. Banker to have so much more money than he has, he rarely thinks to complain about the "unfair" paychecks of Mr. Actor, Ms. Singer, and Mr. Pro Athlete. (Aw, you can't begrudge Mr, Sporty-Pants his wealth. That dude works hard for his money, and you have so much fun watching him play every week! Besides, he's part of your team. He's one of the boys.) It's an enviable position to be in, really. Why would you entertainers want to jeopardize that cushy charmed life by upsetting your adoring public?
Anyway, complain as they may, no-one's going to stop you from saying whatever you like, so go ahead and have at it! You'll just have to learn to live with people saying you should keep your opinions to yourself. It's part of the job-- like the paparazzi. Hey, you wanted to be famous, right...?