Wrong answer. Three reasons:
First, one does not have to be a conservative to recognize the evil of terror. One only has to be sane.
Second, do you understand how loopy you sound when you talk about Hollywood blacklisting libs? That's like the Green Bay Packers blacklisting football players. I'd love to hear your explanation of what would convince these actors that they're gone if they don't run the right newspaper ad.
Third, and most important, this is a dangerous step for them. While terrorists are never going to be fond of anybody from Tinseltown, after the Muhammad cartoon riots and the murder of Van Gogh, these people have to know that they could be made an example. Any public appearance they make is now more dangerous, especially if they go to Europe. Sure, it's nothing compared to the danger that a soldier accepts, but pampered millionaires don't put themselves at risk for the purpose of adding a few more millions.
Be honest with yourself: if these people had run an ad saying "We condemn Israel's murder of Lebanese children" and the DUmmies praised them, would you say "Wow, they're wrong but they have conviction because there are so many Jews in the entertainment industry," or would you dismiss it as the usual Hollywood lib crap?
That would actually explain a lot... ;)
As a cynic, I say, "Follow the money."