Have you ever heard of "irony?" I doubt you have, so look it up.
You telling us how Meryl Streep should live/handle her life as a famous actress, and telling us she's doing it wrong because she's telling others (politicians) how their life "should be done" should ring like a bell when you read that definition of "irony."
First let me be clear that MS and everyone else has the right to say whatever they like.
Next, I'll get into why people like MS bug me. The awards shows they typically do this at are unnecessary. Just because people make movies for the public to pay to see doesn't mean they have to have awards shows at all, much less ones publicized and put on TV. They are nothing but publicity and marketing events designed to gain money and influence for the industry and actors.
What reason is there to believe that an actors opinion on anything other than acting is more valuable than anyone elses? Why should an actors opinion mean more than that of, say, your next door neighbor? They are trying to use the fact that they are a celebrity to influence us on politics despite the fact that the origin of their celebrity (usually) has nothing to do with their knowledge of politics.
Here's the thing though. Consider Political Issue X. People take sides on it but usually it is far from clear which side is correct. Then the actor gets up at an awards show and takes a side. But still it's not clear which side is correct (which of course is a separate thing from from everyone having their own opinion on the issue).
Next consider the issue of whether it is healthy for a society for people to stay informed on political issues and make up their own minds. Unlike Political Issue X, it seems pretty clear to me that there is no ambiguity on this issue. It is clearly better for people to stay informed and make up their own mind. But that very idea contradicts the idea that people should get their political views from celebrities.
If a celebrity really wants to make the world a better place then how about instead of publicly taking a side on Political Issue X, where it's not clear who is right, they take a side on the issue of whether people should stay informed and make up their own minds on political issues? A celebrity doing that would involve them saying something like "I'm proud of my acting work and I hope you keep going to see my movies, but when it comes to politics you should not listen to me or any other celebrity and instead stay informed and make up your own mind."
Why don't they do that? Because it would reduce their own influence, although it would make for a better society.
You may say what's the difference between them doing that and us discussing issues here. The difference is that we're discussing issues here, back and forth, give and take. That is different from using a platform you've been given for non-political reasons and using it to tell other people what to think about Political Issue X and then walking off. And in the process having more influence than you and I and a thousand others put together while not having to face any direct questioning of their publicly expressed views.
That's why I gave a pass before to Schwarzenegger, Franken, etc. At least they entered the fray and took the slings and arrows. The ones that only speak on political issues only when they know they won't be challenged are cowards, regardless of their views.