It's an interesting question you pose, knivesout. I think here in the States, writers and artists are like everyone else in this way: we all want to know what everyone else thinks about public issues, but we are increasingly (many of us, anyway) hesitant to voice our own opinions. This is less true about those on the far ends of the political spectrum, both left and right, of course.
I saw this trend begin around the end of the Reagan years. The radicals don't care what people think of them because of their opinions; everyone else is afraid that there will be some repercussion if they say something unpopular. And, in this day of political correctness, there sometimes are such repercussions. It has become much worse, I've noticed, since 9/11/01. I know I have seen in myself a greater hesitation to say anything that might be considered controversial since then. And I hate that, and have been consciously trying not to be so guarded, just as a matter of principle.
Another aspect of the phenomenon of celebreties in the arts as public opinion leaders: here in the States, there are a huge number of people who seem to think that writers, actors, artists, etc. do not have a right to express an opinion, especially on political issues, because "they might influence someone just because they're famous". Of course, most people don't mind as long as the famous person agrees with them; but let someone that doesn't agree with them express an opinion, and there is general outrage. It seems a bit silly to me.